As President maps his fall campaign strategy, his pollster gives him a boost , and a warning

The President's strategy for helping Republicans in this fall's elections is is beginning to take shape.

* He will likely try to avoid any final, extended barnstorming tour on behalf of key Republican candidates, aides say. Instead, he will confine his stumping around the United States to two- or three-day stints, usually over a weekend.

* Mr. Reagan will continue to rely on his substantial personal popularity to draw votes to Republican candidates.

The President is watching the polls. They show that the majority of voters approve his job performance, still see him in command, and are giving him more time to change the economic course of the nation.

* The President, insiders say, will emphasize this major theme: Where America will be in 1984 if Republican candidates and he continue to receive the public's support. He will thus, as his pollster, Richard Wirthlin, sees it, ''be phrasing the circumstances of the voters' decision'' as he seeks to persuade voters to stay with him and the Republican Party.

Thus, it seems, that the President's campaign effort will be one of sticking fairly close to Washington and talking to the voters via television. At least one major TV address is likely during the late stages of the campaign. The projected theme: what Americans stand to gain by keeping his ''mandate for change'' on track by voting Republican.

* The President sees the importance in this campaign of avoiding the perception of being too political, of being too much involved in the campaign, insiders say.

Thus, he will make a point of underscoring that he is sticking close to the job in Washington - and that he is continuing to hammer away at the big challenges he faces in the economic and foreign-affairs fields.

Meanwhile, Mr. Reagan is getting both good and bad news from his political advisers.

They are telling him, and this is reflected in new polls by Mr. Wirthlin, that by carrying on an effective dialogue with the public, from Washington, he can, indeed, have a very positive effect on the outcome of the fall elections.

The President understands that his job-performance rating has risen in just the last few weeks to about 51 percent. And he now is operating under the assumption, reinforced by Wirthlin counsel, that public patience with his economic program has been extended for another year.

Wirthlin's polls also show that the majority of Americans still blame former President Carter and the Democrats for what is wrong with the economy. So Reagan will thump this warning:

If you move away from backing the Reagan initiative, then the country will sink back into the old Democratic ways of big spending, high inflation, and uncontrollable economic problems.

But the President is also receiving some negative news from Richard Wirthlin and other political advisers. They say:

* That the rising rate of unemployment is causing growing public and, hence, voter concern about his administration.

* That there still are far too many people who believe he is not helping less-fortunate Americans.

* However, more and more questions are being raised by many Americans about the President's ability to cope in foreign affairs.

Thus, in Washington - and not out on the hustings - the President will be trying to ease anxieties in those three areas: By talking directly to the people and by working diligently to take presidential action that will relieve these worries.