DES MOINES, Iowa (AP) _ Illinois Sen. Paul Simon is criss-crossing Iowa this week, capitalizing on his rural roots to boost his bid for the Democratic presidential nomination.

Simon, who announced just last week that he would join the crowded Democratic field, has been making a series of small group appearances in the state that holds the nation's first presidential caucuses next year.

''I've churned butter, I've milked cows, I've baled hay,'' Simon said. ''I understand rural America and rural America's values, I think, perhaps more than any other candidate.''

Simon's candidacy could end next February unless he finishes first or second in Iowa, an aide says. Simon himself describes Iowa as ''crucial.''

''I think that if he doesn't do one or two here, it would be hard to carry the candidacy into the next round of primaries,'' said aide John Fitzpatrick, a veteran Iowa political organizer who helped Sen. Tom Harkin defeat an incumbent in 1984.

Noting his late entry, Simon said, ''I would have preferred to have started two months ago, three months ago, five months ago.'' He had bowed out of the 1988 sweepstakes based on his belief that his friend Sen. Dale Bumpers of Arkansas would run. When Bumpers backed away, Simon was in the race.

''I think it is wide open,'' Simon said. ''If I didn't believe I could sell the people of Iowa on my candidacy, I wouldn't be here.''

Backers said Simon has an advantage in Iowa because he's a top political figure in neighboring Illinois, with media from that state reaching into populous eastern Iowa. His roots are in rural southern Illinois.

However, some Iowa politicians wonder about his strength.

Edward Campbell, a former Iowa Democratic chairman, questioned Simon's ability to finish first or second since other candidates have been busily carving up key constituency groups for months.

''He cannot do that,'' Campbell said. ''The question you have to ask is who is his constituency? His natural constituency would be people like the United Auto Workers. The UAW has already found a home with (Missouri Rep. Richard) Gephardt.''

That union has issued no formal endorsement, but some top officials have joined Gephardt's campaign.

Roughly 13 percent of Iowa's population lives along the eastern border, but Campbell argued that the state's caucus system demands personal contact with candidates and name identification isn't enough.

''These guys keep forgetting this thing is not a primary,'' he said.