Washington — For years after Sinclair Lewis's ''Main Street'' was published, it was quite usual for political writers to go to Sauk Centre, Minn., the inspiration for happenings in that book, to find out what the average American was thinking.
But in more recent years, it has become more fashionable to use Peoria, Ill., as the reference point for such research.
So it was no surprise at all when GOP congressional leader Robert Michel, whose district encompasses the city memoralized by the song ''I wish I was in Peoria,'' was asked, ''What's playing there?''
''I hope that I am,'' said Mr. Michel with a laugh. He is up for election this fall and finds the going a little tough as he copes with voter feeling that the 14- to 15-percent unemployment rate in those parts may be connected with the Reagan-Republican regime, of which Michel is a part.
So he concedes there is deep unrest in Peoria over the economy. ''Notwithstanding,'' he says, ''the people still are willing to give the President more time. They are aware that their problems were not manufactured overnight - that they started a long time ago.''
So Michel is bullish about his own prospects. But he also tends to agree with former GOP chairman William Brock's forecast that House Republicans may yet avoid the disaster at the polls that many experts foresee.
''We are going to surprise people with how well we do,'' he said. ''Of the 52 House freshmen on our side, all except one or two are in good shape - despite that a number come from marginal districts.''
''The Democrats,'' he said, ''simply have not done a good job of coming up with attractive candidates. Instead they have come up with a number of turkeys - who are showing up badly on television.''
Michel says that the President is playing so well in Peoria these days that he has prevailed on him to visit his district in mid-October.
However, Michel admits that the Reagan appeal would be much more limited in other parts of the country. He cites Massachusetts, where he thought Reagan's appearance might be counterproductive for GOP candidates.
Michel separates himself from the President on some issues. He is opposed to the sanctions imposed by the President on companies furnishing equipment for the gas pipeline the Soviets are building. He says the administration should have gotten its European allies to go along before embarking on this undertaking.
The Caterpillar Tractor Company, in Michel's district, was hurt by the administration's pipeline action. And sensitivity to this company's problems may have helped shape the Michel position.
But he says that this company's loss of business as a result of the decision is really ''only a small part'' of problems that have led to big layoffs.
Michel says that the Republicans in Congress have produced legislation that has shifted the direction of the nation. ''A big jolt'' is what he calls it, a move that reflected voter desires in the election of Reagan.
''But how will that shift play in the fall elections?'' a reporter asked. ''Is there still a conservative trend at work among the electorate?''
Says Michel: ''That answer is something that we will only be able to know after the election is over.''