It could still happen. The political upset of the decade. Walter Mondale, defying the polls, edges out Ronald Reagan, and in January is inaugurated as the 41st president of the United States.

Such a scenario looks very unlikely at the moment, but history is full of surprises. For Americans, a Mondale victory would create a torrent of questions, such as:

* What would be President Mondale's top domestic priorities?

* How would Mondale deal with the Soviets?

* Could Mondale really cut the budget deficit?

* Would Mondale slam the door on imports, such as autos, which are hurting union workers?

* Would Mondale support the tight monetary policies that have brought down the rate of inflation?

* Would personal income taxes immediately go up?

Throughout his long campaign, Mondale has made one thing very clear. He has ''profound'' and ''fundamental'' disagreements with the Reagan White House. When pressed by reporters, Mondale says he can find very little that Reagan has done that he likes.

In part, this stems from Mondale's political heritage, born of a progressive tradition in the rural Midwest.

Mondale sees government as a positive, active force that should be used to correct the ills of society. Government is a friend, an ally, of those in need. Government is a means of redressing wrongs, helping those who cannot help themselves, and bringing Americans together for the cause of good.

Mondale has held these views from boyhood. They have not been shaken, even though many American voters have cooled toward big government programs.

As president, Mondale would move quickly to reenergize federal agencies. He is dismayed at Reagan cutbacks and slowdowns in such programs as child nutrition , education, student loans, and assistance for the handicapped. He would push ahead with broadened programs ranging from women's and infants' feeding to impact aid for the cities.

Upon entering the White House, however, Mondale's first actions would probably involve foreign policy rather than domestic programs. Maxine Isaacs, press secretary and close adviser to Mondale, says that, without doubt, his top priority on the day he took office ''would be to move toward arms control in some way.''

In several speeches this fall, Mondale has made clear the importance he attaches to halting the nuclear arms race. In Los Angeles last month, for example, he said:

''On the day I am inaugurated, I will tell the Soviet leader, 'Meet me in Geneva within six months to negotiate a mutual, verifiable nuclear freeze.' ''

Mondale argues that Reagan is failing to provide courageous leadership on the arms issue, which he calls the most crucial issue of our time. Specifically, he makes a contrast between Reagan, who has not negotiated a new arms treaty with the Soviets, and John F. Kennedy.

''A generation ago, President Kennedy broke the deadlock on atmospheric testing by declaring a pause in our testing - and challenged the Soviets to join us. They did. We sat down, and two months later the test ban treaty was signed.''

There was a risk in what Kennedy did, Mondale will concede. He took unilateral action to demonstrate that the United States was serious about arms control. But Mondale says it was the kind of gesture that is needed today to get arms talks going once again.

Mondale has vowed, in the Kennedy tradition, to halt testing of nuclear and space weapons for up to six months in an effort to reach a new agreement with the Soviets. He makes clear that if the Soviets rejected his initiative out of hand, or if they simply ignored it and kept on testing their own weapons, the US would also resume testing. But it's an effort that Mondale says can demonstrate ''the peace-building power of presidential leadership.''

Domestically, Mondale has indicated that his first actions will be on two fronts: tackling the current $170 billion federal deficit, and restoring some of the cuts Reagan has made in social programs.

Those two broad areas - arms control in foreign policy and budget deficits at home - are priorities No. 1 and No. 2 for a Mondale White House. But Mondale has spelled out in fine detail dozens of other positions that in some cases could significantly change the direction of government, industry, education, women's rights, the environment, and foreign trade, to name a few.

At the heart of Mondale's deficit-cutting program are higher personal and corporate taxes.

Proposing higher taxes in the middle of a presidential election flies in the face of conventional political wisdom. Robert Strauss, who heads a group of Democratic pros trying to help the Mondale campaign, concedes the tactic worried him.

''When (Mondale) said, 'I'm going to raise your taxes, and I'll tell you so and Reagan won't,' had it been anyone but Mondale, I'd have thought it was a mistake,'' says Mr. Strauss.

''It's gutsy and it's full of risks, but it's worthwhile. ... Mondale had good reason because he was seen as being too cautious.''

Even so, one is left with the impression, after several conversations with Strauss and other top Democrats, that Mondale's tax gambit probably didn't help the ticket very much, if at all. Many Democratic candidates for Congress have run away from the plan, which would put a bigger tax bite on any family making more than $25,000. Today, with many two-income families, that includes millions of voters.

The other half of the Mondale budget program involves spending - both decreases and increases.

Military spending would be trimmed by $25 billion, including an end to the B- 1B bomber program, the MX missile, and Reagan's ambitious strategic defense project, nicknamed ''star wars.'' Some military spending would be increased, particularly for conventional forces and sustainability (bullets, rockets, grenades, etc.).

Health-care outlays would be reduced $12 billion a year by capping the federal budget in this area. Farm support programs would be cut $4 billion by doing a better job of balancing supply with demand.

Mondale's greatest cost cutting, however, would be $51 billion a year in interest expenses, most of which he claims could be saved by reducing the federal deficit and thereby reducing the strain on credit markets.

Some of these cost savings and higher taxes would be offset by spending an additional $30 billion for social programs. These programs, such as food stamps and student loans, have borne what Mondale charges are ''the brunt of Mr. Reagan's unfair budget cuts.''

To a large extent, for better or for worse, many of Mondale's positions on such issues reflect his political marriage to major organizations and constituencies in the United States.

This close alliance with large groups, such as teacher organizations, has been at the same time both Mondale's greatest political strength, and his greatest liability. One of the most obvious of these Mondale allies is the AFL-CIO. Big labor swung its support behind Mondale early in the political season, shored up his strength after his loss in New Hampshire to Sen. Gary Hart , and gave him the extra boost he needed in states like Illinois, Pennsylvania, and New Jersey to lock up the Democratic nomination.

Some analysts argue that Mondale's close ties to labor, teachers, feminists, and other groups have hurt him badly. It creates an image of a politician subservient to special interests. Yet Mondale's friends say that those who make this charge overlook reality.

Arthur Naftalin, a Mondale friend and former mayor of Minneapolis (1961-69), is one who likes the ties to labor.

''The fact that Mondale has reached out to this wide range of what they call special interests - labor, the teachers, old people - I think that is a positive ,'' says Dr. Naftalin. ''After all, that is a way you build a coalition to govern this country.

''I think Mondale's got the proper elements in the coalition. ... I think that he's got on his side the elements of America that I don't think Reagan has. ... He's got the elements of bedrock America. ... I think that the trade unionists, older people, minorities, and so on through people in the education world - this, I think, represents the big body of Americans. He'd be in a pretty good position to govern.''

Ms. Isaacs says that while Mondale has been criticized for bringing so many groups into his tent, that is actually a plus for the Democrats.

''When we were going through the primaries, the pundits were writing that he'd torn the party apart, that it could never be brought together. Yet we went into the most unified convention in history. Now we have as unified a Democratic Party as we've had in 24 years,'' she says.

''That is a tribute to Mondale's political strength, to to his ability to reach out. He's sort of relentless. He pursued (Gary) Hart very hand and he brought him in. He pursued (Jesse) Jackson very hard and brought him in,'' Ms. Isaacs notes.

''The critics don't understand. Mondale's best political skill is bringing people together who were totally at war a month before.''

Yet Mondale's critics persist. They wonder if Mondale can stand up to those who have rallied around him, like big labor. Can he resist pressures to impose import quotas, for example, to protect union jobs, even though that might reheat inflation?

Former Sen. Eugene McCarthy, a fellow Minnesotan, is one of those who doubts Mondale is right for the job. Although he is considered to be a liberal Democrat , Mr. McCarthy backed Reagan in 1980. And his sympathies lie the same way in 1984.

McCarthy suggests that Democrats are retreading old ground, resurrecting outworn ideas, and endangering progress against inflation.

McCarthy, who takes a caustic view of the Mondale campaign, says that putting Mondale at the top of the ticket shows a ''certain boldness'' by Democratic leaders ''because they are taking their weakness right into the game.''

He notes that Mondale has no way of running away from issues like Vietnam (Mondale backed the war for years), indexing of government salaries and pensions (McCarthy worries about what that can do to inflation), budget reforms in Congress (''ridiculous''), and a reversal of the process of democratizing party rules (''Carter-Mondale tightened the rules and made them less democratic.'')

A free society, says McCarthy, can get into serious trouble in two ways: a prolonged war like Vietnam, and economic disorder such as runaway inflation.

It's inflation, he says, that recent Democratic politicians have found most difficult to deal with. In this regard, he argues that the problem with Mondale is that he acts as an advocate of narrow groups rather than as a leader of the whole nation.

''His approach to some of these groups, teachers and so on, is, 'I don't really agree with everything they want, but I told them I would get as much as I could,' '' McCarthy suggests.

What are some of the programs Mondale has laid out to capture support from the unions, teachers, nurses, the elderly, feminists, Hispanics, blacks, and others? Here, in very brief form, are some of the major ones:

Foreign trade. America buys more than it sells abroad. That costs jobs at home - nearly 3 million jobs, Mondale says. His solution includes several parts. First, reduce federal deficits that Mondale says hike interest rates. These high rates inflate the value of the dollar, and make it harder to sell US goods abroad. He also favors more support for the Export-Import Bank, which boosts sales of American goods abroad. Mondale has also spoken in favor of laws which would require that cars sold in the US contain a certain percentage of American-made parts - a law that critics such as Sen. Gary Hart claim could boost prices.

Women's rights. Two issues separate Mondale sharply from Reagan. Mondale supports the right of free choice for women on abortion. He also supports the Equal Rights Amendment. Mondale favors a national effort to reduce abuse of children and spouses, enactment of tough fair housing laws to protect families with children, and elimination of discrimination against women in pensions and insurance. Pay equity is another Mondale priority. He supports equal pay for equal work - but also equal pay for work of ''equal value'' - a concept that is more controversial.

Civil rights. Mondale would prohibit tax breaks for segregated private academies. He would renew the push for school integration and fair housing with ''vigorous enforcement of all law and court decisions.''

Budget priorities. Mondale would support higher spending for child nutrition, women's and infants' feeding, food stamps, student loans, and help for the handicapped. He would boost outlays for research into health problems of the handicapped and expand medicaid funding for child health and pregnant mothers. He would also boost spending for legal services which assist disabled persons to ''obtain rightful benefits.''

Education. A five-point program highlights the Mondale strategy. Each involves a federal investment. The program includes: 1. A ''fund for excellence, '' costing $4.5 billion a year, to support school improvement programs at the local level. 2. Better pay for teachers to attract a better quality of personnel. Cost: $1 billion a year. 3. Another $1 billion per year to improve US school laboratories, libraries, and graduate facilities. 4. About $3 billion a year to help minority and disadvantaged children. 5. About $1.5 billion a year for student assistance programs and to aid black colleges.

Environment. Mondale charges that Reagan has failed to enforce the laws already on the books. He would ''return to full enforcement.'' Top priority would go to dealing with problems of toxic wastes and acid rain.

Energy. Move quickly toward greater energy security for the United States by supporting innovative technologies and new energy sources, more conservation, and faster filling of the Strategic Petroleum Reserve.

Jobs. Give special attention to youth and others with chronic unemployment problems. Use public works - roads, bridges, and other projects - to supplement job opportunites in the private sector. Encourage private training of workers through special tax credits.

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