Dallas — The President's political prospects for the 1984 election look so good right now that one is reminded of a favorite Reagan line, which goes something like this:
''As President Tom Dewey once told me, you have to beware of overconfidence.''
Guarding against overconfidence has become a watchword among Republican planners this year. But they can hardly hold back the smiles. The polls, both public and private, give the President a 10- to 15-point lead. He's ahead in every region of the country - even the Northeast. Mr. Reagan is personally popular. The economy is sizzling. Inflation has cooled. The nation is at peace. And Democrat Walter F. Mondale finds his campaign bogged down with questions about the finances of his running mate, US Rep. Geraldine A. Ferraro.
The mood is so positive here that some Republicans talk about extending a Reagan victory into the Congress. A pickup of 15 to 25 seats might help tip the balance in the US House back toward a controlling coalition of Republicans and Southern Democrats.
The experts, however, remain cautious with election day still more than two months away.
Republican political sage Lyn Nofziger, having breakfast with some reporters here, says two principal things worry him right now.
First, there are still a lot more Democrats out there than Republicans, he observes. The latest Gallup poll gives the Democrats a 42-to-28 margin in party preference. The other 30 percent are independents. That means, says Mr. Nofziger , who is an unpaid adviser to the Reagan-Bush campaign, that the President needs support from at least 20 percent of all Democrats to win. He's currently expected to get at least that many - but Nofziger worries anyway.
Second, Nofziger observes that the President has ''attempted to make a drastic change in the direction of government.'' That effort has created strong, ''vindictive'' opposition - even more than Richard Nixon faced prior to Watergate. This energized opposition from feminists, union leaders, and others could be damaging, Nofziger says.
But political scientist Tom Cronin suggests that Reagan ''would have to throw it away'' to lose this year.
Dr. Cronin, an active Democrat who backed Sen. Gary Hart for his party's presidential nomination this year and once worked for Mr. Mondale, points out that Reagan has three things going for him that should make him virtually unbeatable: his popularity, the strong economy, and the absence of a war abroad.
Cronin, who co-wrote a best-selling college political-science textbook, says hope for the Mondale-Ferraro team lies in focusing on the four things that most concern voters: arms control, military spending and waste, reform of the tax system, and the future of social security and other entitlement programs.
What should concern Republicans, Cronin says, is that in forthcoming debates, Reagan and Vice-President George Bush may fail to address these public concerns about the future.
''If ... the Republicans seem tired, worn out, and just full of rhetoric,'' Cronin says, Mondale could score a breakthrough.
GOP chairman Frank J. Fahrenkopf Jr. says the party's planners are well aware of this problem. Republicans will frequently compare the Reagan-Bush record to the Carter-Mondale record during the campaign, Mr. Fahrenkopf says; but the President will also focus heavily on his ideas for the future.
The GOP mood here in big, booming Dallas belies any serious concern about a loss this year. This city of almost 1 million, built on money from oil, banking, insurance, agriculture, and fashion, symbolizes Republican hopes for their own party as well as the country itself.
Just three blocks from the convention center, a new glassy skyscraper that is going up has reached its 70th floor. At downtown sites nearby, a dozen construction cranes rumble away over new projects. Outside the city center, prosperous suburbs spread for miles across the flat, hot land in all directions.
This is a city, says one resident, where people ''go for it.'' Many fail. But they try again. And many succeed - and succeed big. Dallas epitomizes the kind of ''can do,'' free-market energy that Reagan and staunch supporters like Sen. Paul Laxalt (R) of Nevada hope to release through lower taxes and less regulation.
Republicans such as Senator Laxalt argue that the current economic expansion has only scratched the surface of the growth and prosperity that can be achieved by reducing the intrusion of government into American life.
Democratic strategists counter that this Reagan-Republican prosperity has been unfair to many Americans - not just those on welfare, but middle-income people as well, and, they say, through federal deficits, prosperity has been bought for today only to be repaid by future generations. It is those Democratic arguments about ''fairness'' that worry Republican planners like Nofziger, because they could galvanize anti-Reagan voters.
Today only 110 million of 170 million potential voters are even registered. Of the 60 million unregistered voters, two-thirds are believed to be philosophically similar to Democrats, observes Tony Harrison, a registration official at the Democratic National Committee.
Democrats are hoping to get as many as 5 million of these Democratic-leaning potential voters added to the rolls by November - enough, they hope, to tilt the election in several close states. In addition, unions that back Mondale are conducting registration efforts of their own that could swell that number.
The GOP, mindful of the growing numbers of Democratic voters, are active in their own efforts. The national committee has a target of 2 million new voters - and currently estimates that 1.5 million have been added to the rolls in key states like California, Illinois, and Michigan. The Reagan-Bush campaign hopes to sign up another 2 million Republicans through its own voter identification program.
All in all, the election outlook here is upbeat. Even so, Nofziger expects Reagan to hit the campaign trail longer and harder than projected a few months ago. The White House is apparently listening to that advice from ''President Dewey.''