Imagine this wildly implausible scenario: Stung by the Geraldine Ferraro challenge, Ronald Reagan decides to dump George Bush as his vice-presidential running mate and pick a woman. The contest is on between Sen. Nancy Kassebaum of Kansas and Transportation Secretary Elizabeth Dole. Then a floor fight develops over the GOP platform. Women delegates demand reintroduction of a plank favoring the equal-rights amendment. Sen. Robert Dole of Kansas leads a battle for a plank calling for a tax increase in 1985. And a group of Republican moderates work the floor to gain support for a plank urging adoption of a comprehensive nuclear test-ban treaty.

As the platform feud grows acrimonious, some delegates begin to murmur that Mr. Reagan is too far right after all. At which point George Bush, smarting from being shunted aside, decides to make his move. The presidential nomination challenge is on.

All fantasy, of course.

As the Republicans gather in Dallas for their 33rd quadrennial convention Aug. 20-23, the event is so devoid of suspense that the biggest problem for planners is how to inject a bit of zest into what promises to be an uneventful nomination of the party's presidential candidate for 1984. Wanted: drama without conflict

The Democratic convention in San Francisco has made Republicans especially sensitive to their own party show. Contrary to expectations, the Democrats managed to resolve the deep divisions within their party, pull themselves together, and emerge with their troops fired up over an historically unprecedented Democratic ticket. It was not only a good show, but it was exhilarating as well.

Not that Republican planners want any surprises. Four years ago the situation was in some respects like that at the recent Democratic convention. Ronald Reagan was assured the nomination in Detroit, but he needed to broaden his base, fold in the centrist and moderate factions of the GOP, and emerge with a united party. He visited with unemployed Michigan workers. He met with women over the withdrawal of the equal-rights amendment (ERA). He also generated a late-night television drama by considering Gerald Ford to be his running mate. That fell through when Mr. Ford insisted on more executive authority than Reagan was willing to grant.

No such problems exist in 1984. The party has its disparate ideological voices - and a crop of ambitious Republicans with an eye on 1988 - but the priority now is to get President Reagan reelected. The convention objective will be to convey the message of a party unified under the conservative leadership of Ronald Reagan.

''We're as unified now as (the Democratic Party under) FDR was in 1936,'' says a Reagan-Bush campaign official.

While unity is expected to be the hallmark of the orderly, well-programed convention seen on television, the GOP is not without intraparty disagreements that could produce some lively, even discordant moments. There are divisions over the party platform. Moderate Republicans are challenging platform planks on such issues as taxes, arms control, women's rights, and the environment. Right-wing elements, for their part, would like more conservative language on fiscal policy. Party liberals decry one-sidedness

The 106-member platform committee, however, is controlled by young conservatives, and the White House has kept a tight rein on fashioning of the platform. After the Republican Party failed to hold the traditional round of regional platform hearings across the country, a group of GOP liberals and moderates recently held an informal hearing in Washington to take testimony from environmentalists, civil rights activists, nuclear freeze advocates, and others.

''We're used to a more open process,'' says Sen. Charles McC. Mathias Jr. of Maryland.

''There's a perception that we've closed the process to the rank and file. I'm concerned that if you take politics seriously and the Democratic Party is seen to be increasingly liberal and the Republican increasingly conservative, that's not good for our society,'' says Kenneth Ruberg, director of the Republican Mainstream Committee, a group formed recy ly to advocate more moderate view4- hin the party. ''Republican constituencies are basically moderate, yet conservatives are in control of many party organizations.' Platform could produce some sparks

The platform will be hammered together in Dallas this week. Seven or eight subcommittees will hold open hearings before starting the mark-up process. Various changes could be made. Senator Dole, chairman of the Senate Budget Committee, opposes writing a ''no tax increase'' promise into the platform. Republican omen are seeking stronger statements on women's rights, including tightening of laws on sex-based discrimination. (Some actually want a pro-ERA plank, but will bow to political realism.) Sen. Lowell P. Weicker of Connecticut favors a strengthened commitment to civil rights.

Contention over the platform could spill onto the convention floor in debate over minority resolutions. But present party rules make that difficult. Reagan campaign officials expect the platform package to be locked up and printed by the time the convention opens. They say they do not anticipate Cny floor skirmishes. Some GOP moderates hope there will be none. Concern over a tilt toward West

Still another move is afoot that involves the rules committee. Some Republicans are concerned that the West and the Sunbelt states today are overrepresented in the convention because of the ''uniform victory bonus'' adopted in 1924. This rule gives three extra delegates to any state in which the Republican presidential candidate won, regardless of the state's population. As a result, say critics, the convention is tilted too much toward the conservatives.

This year the Republican Party has formed a new rule stating that no state shall have fewer delegates than delegations from Puerto Rico or the District of Columbia. ''This, too, will serve the Western states better,'' says Jayne Hart, executive director of the Ripon Society, a liberal Republican organization.

With a view to providing a better balance in 1988, national committeeman James T. Neal of Indiana is proposing a change in the delegate-selection formula. The proposal would add 1,076 bonus delegates apportioned among the states, based on the total vote for the GOP presidential ticket in the past three elections. It would also reduce the numby4f alternate delegates by half.

Some political observers suggest that the Reaganites, in trying to have a totally orchestrated convention, add to the GOP image problem. Republicans are often perceived as a homogeneous, white-male-dominated party without the kind of ethnic diversity that lends spice and vigor to the Democratic organization. This year the Republicans have made massive efforts to make sure there are more women delegates. In 1980, only 27 percent were women; in Dallas, women will comprise 45 percent of the delegates. But there will be fewer than 100 black delegates out of a total of 2,235. Hispanics will comprise about 10 percent. Hard line toward Soviets promised

Along with exuding unity and efficiency, the Republicans want to set the dominant themes of the fall campaign. Party officials say the Democrats in San Francisco conveyed a ''gloom-and-doom outlook'' about the American economy and an unrealistic view of the Soviet Union.

They intend to respond by pointing to the President's record in turning the economy around and in bolstering America's image abroad.

''Unlike the Democrats, you're going to hear the word 'Soviet' used,'' says a Reagan-Bush '84 operative. ''In prime-time television they referred to the Soviet Union as simply a special interest and nothing more than that. No mention of its role in destabilizing Central America and of the fact that they left the bargaining table.

''So the convention will not be bashful about pointing out the reality of the world.''

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