Black ties and fancy fund-raisers are out, and western wear and ethnic events are in for Ronald Reagan, at least for the duration of the 1982 election campaign.

In an effort to dispel the impression that Reagan is "a rich man's president," his advisers have reverted to the basic themes and backdrops of his successful presidential campaign. Reagan dressed in western casual at a Los Angeles soundstage "roundup" for California's U.S. Senate candidate Pete Wilson, an event that had all the western spontaneity of a minuet. He will do a fund-raiser, sans tuxedo, for Rep. Millicent Fenwick of New Jersey, who also is running for the Senate, on the same day both of them appear at an ethnic event that will allow the multilingual Fenwick to demonstrate her fluent Italian.

For other candidates, Reagan will eschew fund-raisers entirely, turning over the money-raising chores to Vice President Bush and members of the Cabinet. Reagan will be cast not as partisan fund-raiser but as president, cowboy and natural man. It was a strategy that worked for him in 1980 and that his advisers believe will work for him again.

The principal reason this image-making succeeds is that Reagan really is a natural man who prefers plain food to fancy dinners and who would rather spend a day working or riding on his secluded ranch than poring over papers in the Oval Office.

Still, little is left to chance. Most of the pictures that photographers have been allowed to take during this vacation trip show Reagan dressed in ranch clothes, which still fit him as naturally as they did when he learned to ride nearly 50 years ago. Putting it in perspective, an adviser commented: "The cowboy is still a hero to Americans. The only people who care for rich Republicans in tuxedos are other rich Republicans."

Flexibility is the word on the Reagan campaign schedule this fall, as strategists seek to husband the resources of the White House and use the president where he can make a small but vital difference.

Reagan is committed to appearances for Fenwick and Utah Sen. Orrin G. Hatch. Senate races in Virginia and Maine also have high priority, and Reagan will do what he can to help the gubernatorial and Senate nominees in Texas and California. He is likely to campaign for Lewis Lehrman in New York if Lehrman wins the gubernatorial nomination and the polls show he has a realistic chance of also winning on Nov. 2.

As in New York, it will be the late public opinion surveys that will determine where Reagan campaigns. His strategists have set aside two or three days in each of the final three weeks of the campaign so the president can speak where his presence will be useful and avoid those states in which Republican candidates are either home free or have no chance to win.

The latest survey by White House pollster Richard Wirthlin's Decision Making Information firm shows Pete Wilson leading Democratic Gov. Edmund G. (Jerry) Brown Jr. by seven points in the California Senate race, and Republican gubernatorial nominee George Deukmejian trailing Los Angeles Mayor Tom Bradley in the battle to succeed Brown . . . Arthur Finklestein, third pollster for the White House after Wirthlin and Robert Teeter, has proved a comet in the summer skies. Finklestein lost favor with his in-house sponsor, deputy chief of staff Michael Deaver, for letting it be too widely known that he, Finklestein, was the brain behind Reagan's "spontaneous" visits to schools and black families. Spontaneous visits are supposed to be spontaneous. The final straw, apparently, came after Texas Gov. Bill Clements complained that Finklestein had released polls showing him trailing Democratic nominee Mark White. Now the White House is back to two pollsters.

The president is scheduled to send an omnibus crime message to Capitol Hill soon after he returns to Washington from Santa Barbara. It will put in one document Reagan's various advocacies for tougher law-enforcement measures, including modification of the controversial exclusionary rule and, possibly, a recommendation that the verdict of "not guilty by reason of insanity" be changed to a verdict of "guilty but insane" . . . On Sept. 9, Reagan is scheduled to be an Alfred M. Landon speaker at Kansas State University in an event that will honor the 1936 Republican presidential nominee. Past speakers include presidents Nixon and Ford, Alexander M. Haig Jr. and -- in 1967 -- Ronald Reagan, who then gave a controversial speech about higher education at a time when he was one of the principal voices in the land calling for a crackdown on student demonstrators.

Describing the joys and burdens of the presidency at a political fund-raiser in Los Angeles, Reagan said last week: "You go to work in an elevator and you go home in an elevator and sometimes you get a little claustrophobia. The quarters are beautiful and it's very fine living and all that. But every once in a while you do look out the window and see people walking by and you say, 'You know something they can do and I can't -- I can't just walk down to the corner drugstore and pick out a birthday card or a magazine or something.' And then you go to Camp David and get it out of your system."