President Reagan criss-crossed the Midwest today, warning against overconfidence and trying to convert his popularity into Republican gains in the Democratic-controlled House.

"Don't send me back there alone," Reagan said to thousands of enthusiastic supporters at flag-waving outdoor rallies in Saginaw, Mich., and Cleveland.

In Cleveland, Reagan called for the election of Republican House challengers to "keep the pressure on House Speaker Thomas P. Tip O'Neill" and blamed House Democrats for delaying anticrime legislation.

After a rally here in which he extolled the virtues of Republican Sen. Charles H. Percy, Reagan, unaware that the sound system was still working, said, "Look at that brave bastard out there with the Simon sign."

Rep. Paul Simon is Percy's Democratic opponent in what is regarded as a neck-and-neck Senate race. Percy laughed loudly in response to Reagan's remark.

Reagan strategists say Reagan is securely ahead in the three midwestern states -- Michigan, Ohio and Illinois -- that he visited today. Except possibly for Percy, who is to get another boost from Reagan in a Chicago speech Sunday, these strategists say there is little more Reagan can do to help Senate candidates.

So in the final weekend of the campaign, Reagan is shifting his emphasis to the House, where GOP gains could ease the way for Reagan's budget and legislative program in 1985, assuming he is reelected.

In Cleveland, Reagan blamed the House Democratic leadership for blocking legislation to establish special tax zones for businesses in distressed urban areas. He also cited the three-year delay in the crime bill passed by Congress before adjournment this year as an example of the "strong obstacles" posed by House Democrats.

"With a lot of effort, we finally got this important legislation through the Congress, and now it's law," Reagan said. "It's this kind of thumbing of nose at our citizens that makes me believe we will find out this year exactly what we found out in in 1980 -- in the United States of America, the people are in charge, not the Democratic leadership of the House of Representatives."

Despite this new appeal, Reagan continued to insist that he does not seek a partisan victory. He reminded audiences that he had once been a Democrat and again drew contrasts between Democratic presidents Harry S Truman and John F. Kennedy and his challenger, Walter F. Mondale.

Repeatedly, Reagan jabbed at Mondale on economic and national security issues, depicting him as a candidate of despair who consistently has supported tax increases and voted against a strong defense.

"My opponent may take a negative view of America, but he'd better not try to peddle his doom and gloom in Michigan," Reagan said at the Saginaw rally in an airport hangar decorated with patriotic bunting and gigantic American flags.