Until now, the Democrats have held the spotlight in the maneuvering for the 1988 presidential election. But that is about to change. The two leading Republican contenders, Vice President George Bush and Senate Minority Leader Bob Dole, are announcing their candidacies in the next few weeks. And the recognition is growing in the political community that odds favor the Republicans' nominating the next president.

One important psychological factor -- the ''time for a change'' theme -- could help the Democrats. But ''objective conditions,'' as Marxists like to say, give Republicans the edge.

The nation is at peace, no vital interests of the United States appear to be in jeopardy, and President Reagan is on the verge of signing the first arms-control agreement in eight years. The summit meeting with Soviet leader Mikhail Gorbachev, to be held in this country, cannot help but boost Reagan's standing in the polls.

The nation is also in its fifth year of steady economic growth without inflation. The president has just approved a bipartisan congressional mechanism for deficit reduction which may ease the largest threat to the future of the economy.

Nothing is certain 13 months before an election, but trends are moving in the GOP direction. Between June and September, the number of people in The Washington Post-ABC News poll saying that things in this country ''have gotten pretty seriously off on the wrong track'' declined from a record 62 percent to 54 percent. That's still not terrific, but it probably reflects a rise in economic confidence and the easing of gloom induced by the Iran-contra affair and its disillusioning effect on Reagan's leadership.

The impact of these changes is reflected in the poll question about which party has the best chance of winning the White House next year. Voters give Republicans a 10-point edge, 52-42 percent.

One additional reason Republicans enjoy that early favorites' role is that Bush and Dole are the only people among the expected candidates of both parties that a majority of all voters consider ''basically qualified'' to be president of the United States.

Bush has earned that degree of confidence from 80 percent of the registered voters in the survey; Dole, from 74 percent. Seven out of ten Democrats in the sample give them that accolade.

Only 54 percent of the Democrats (and 44 percent of all registered voters) put the ''basically qualified'' label on the leading Democratic contender, Jesse L. Jackson. As time goes on and the field is winnowed, the ''leadership gap'' will diminish. The majorities who now say they don't know enough about the other Democratic contenders to judge their qualifications will almost certainly develop positive impressions of the survivors. But all the reporting I've done for the past year underlines the importance voters attach to competence and professionalism in the next president. And history shows how difficult it is for newcomers to the national scene to get over that barrier of credibility and confidence.

Of the seven presidents since Franklin D. Roosevelt, only Jimmy Carter was not well known to the American people a year before his election. And Carter lingers in many voters' minds as a warning of the risks of putting an untested stranger in the Oval Office.

Aside from the ever-present danger of the economy or the world situation turning sour, the main reason for caution about predicting a Republican victory is the evidence of a ''time for a change'' psychology developing in the country. The Post-ABC News poll asked respondents to choose between these statements: ''After eight years of Ronald Reagan, we need a president who can set the nation in a new direction,'' or ''We need to keep the country moving in the direction Ronald Reagan has been taking us.''

By a 55-41 percent margin, registered voters opted for a ''new direction.'' Three out of 10 Republicans chose that answer, as did eight out of 10 Democrats.

One key question for the next few months, therefore, is whether Republican candidates can plausibly suggest ways in which their administrations might differ from, or go beyond, the Reagan agenda. Such trailing GOP hopefuls as former Delaware governor Pierre S. (Pete) du Pont IV and Rep. Jack Kemp have been far bolder than Bush or Dole in drawing their own designs.

But Bush and Dole have started to try. The vice president has suggested he sees a more activist role for government in the education and environmental areas. And the Kansas senator talks about programs that would bring help to Americans who are not sharing in the general prosperity.

These are tentative moves, and both men told me in recent interviews they thought Reagan had essentially set the nation on the right course. If they appear as status quo figures, they could lose.