Reagan and Bush flap coattails for Congress candidates; Here's why the President has held lead for so long

As President Reagan heads into the campaign homestretch with a three-stop blitz in California today, political experts see these factors working for a decisive Reagan victory:

* The campaign has revolved not around the ''issues'' but around the personalities and leadership images of the candidates. Mr. Reagan remains an extremely popular leader, although voters do not support many of his policies.

* The American public is not looking at the future but at the present, and many Americans feel in a good, or at least better, mood. Reagan has not even addressed what he would do in a second term, reinforcing that this election is a referendum on the last four years, not approval of a known agenda for the next four.

* The circumstances in which the election takes place are favorable to the President. Despite some worrisome glitches - and the long-term problem of gargantuan budget deficits - the economy is performing well, with inflation, unemployment, and interest rates down. There are no crises abroad in which the United States is embroiled.

Except for a momentary surge toward Democratic challenger Walter Mondale after the first presidential debate, public support for the President has remained high since the GOP convention, leading some analysts to conclude that the vote Tuesday is likely to be more a positive one for Mr. Reagan than a negative one for Mr. Mondale.

''The steadiness of support for Reagan and the relatively small number of undecided votes have been unbelievable,'' comments Stephen Wayne, a presidential scholar at George Washington University and a Democrat. ''No one could have beaten those ratings. There is a general feeling that he has improved things, that he's a decent leader, and people have decided he's earned another term,'' he says.

Everett Ladd, an opinion expert at the University of Connecticut, also points to the fairly consistent high level of support for Reagan all year long. Historically, he says, whenever a personally popular president has run for reelection and the economy has been performing well and the general direction of leadership is one that people endorse, he has never lost.

''Mondale is a good politician who has been dealt a bad hand,'' he comments. ''Anytime an incumbent is running for reelection, it has to be a referendum on his record and that is the case this time as it was in 1980.''

Throughout the campaign Mondale has sought to draw out the President on his intentions in the next four years. Most economists say serious problems lie ahead because of the widening budget and trade deficits. There is also an array of foreign policy challenges - arms control, the Middle East, Central America. These are logical issues for a discourse in an election year.

But the President has conducted a campaign built largely around images and slogans (''Peace through strength,'' ''America is back,'' the Democrats' ''gloom and doom'') infused with optimism and tied to the record of the past four years. Even his poor performance in the first debate and less-than-dazzling performance in the second were not enough to turn the public focus of the campaign onto substantive questions.

Yet, paradoxically, there continues to be a gap between the public's support for Reagan and approval of his policies. According to a Gallup survey conducted at the end of September, 50 percent of voters favor a reduction in defense spending, with 46 percent opposed (4 percent no opinion); 74 percent favor an increase in spending on social programs, such as education and medicare, with 24 percent opposed; 63 percent support an equal rights amendment; 75 percent, a verifiable nuclear freeze; and 64 percent, cost-of-living-adjustments on social security benefits. A high 64 percent oppose any relaxation of pollution controls.

''On a whole host of issues he doesn't have the American people with him,'' says presidential expert Thomas E. Cronin, ''and in terms of educating people the election has been a failure. But most people vote retrospectively not perspectively. And they're rewarding him for the fact that things are greatly improved and they feel comfortable with him.''

''At the presidential level people are revealing themselves as status-quo oriented,'' says Nelson Polsby, a political scientist at the University of California, Berkeley. ''People say they will vote for him though there is no big endorsement of the specifics.''

Because polls show Reagan winning reelection by possibly landslide proportions, most political analysts now are focusing on the Senate and House races, as well as races for state and local offices, waiting to see what the election signifies in terms of a political realignment. There already is what Dr. Ladd calls a ''schizoid realignment,'' with Republicans the majority party at the presidential level and Democrats still enjoying majorities in the House of Representatives and holding many governorships and local offices.

If the Reagan coattails are short and Democrats do well at other levels, say experts, it can be concluded that voters went for Reagan because (1) he has boosted public morale and fostered a ''feel-good presidency'' and (2) the complex Democratic nominating process may not have brought forth the most attractive candidate.

Buoyed by the polls, Reagan has been promoting Republican candidates for Congress, especially the House. The Republicans are expected to keep control of the Senate. But the GOP needs to regain at least 25 House seats if the President , assuming he is reelected, is to put together the coalition of Republicans and conservative Southern Democrats that enabled him to push through his economic and defense programs.