Returning members of Congress of both parties said yesterday that high interest rates are choking the economy and scaring their constituents, and Republicans said President Reagan needs to act quickly if he is to save his economic plan.

"Within 90 days, something's got to give," said House Minority Leader Robert H. Michel (R-Ill.), referring to the 20 percent interest rates. "I'm a political creature, and our political futures require there's got to be some movement before spring."

While Michel insisted that he had "more faith in the administration than in Wall Street," Democrats and some rank-and-file Republicans said that the slumps in the stock and bond markets in the month since Reagan's budget cuts and tax cuts were passed had shaken public confidence in the White House economic program.

Rep. Morris K. Udall (D-Ariz.), reflecting the cheerfulness many of the Democrats feel at the turn of events during Congress' August recess, said at a breakfast with reporters, "I flatly predict the Democrats will win 15 seats in 1982 on the economic issue alone."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said the Republicans' promise that deep spending and budget cuts would quickly invigorate the economy was collapsing "like a house made of cards."

Few Republicans would go that far, but from California to Mississippi, conservatives in both parties who had supported the Reagan requests said high interest rates were causing serious problems for builders, realtors, auto dealers, small and big businessmen and consumers in their districts.

"Nobody can live with 20-22 percent interest," said Rep. G. V. (Sonny) Montgomery (D-Miss.), one of the "boll weevil" Democrats who gave Reagan crucial support on the budget and tax votes. "On a crisis scale of 1 to 10," said Rep. John H. Rousselot (R-Calif.), a staunch Reagan backer on the Ways and Means Committee, "I'd say we're about 7, and climbing."

Though inflation has fallen somewhat in the last few months, interest rates have not. This is generally ascribed to skepticism in the financial markets as to Reagan's ability to hold the deficit next year to his target of $42.5 billion and to achieve a balanced budget by 1984.

Some Democrats had warned that the 25 percent, across-the-board tax cut over the next 33 months that Reagan pushed through Congress this summer in a stunning display of political power would bring big deficits in its wake. They claimed early vindication.

Rep. Andy Jacobs (D-Ind.), who cast a vote for the Reagan budget cuts but not the tax cuts, said Indianapolis businessmen are beginning to "think that freshly printed money may be ashes in their mouth."

But Rep. Charles W. Stenholm (D-Tex.), a leading "boll weevil," said voters in his district are still "very supportive" of both the budget and tax cuts. Like everyone else, however, Stenholm said, "We just can't live with those interest rates." In a farm district like his, he said, the combination of high interest rates and "the bottom falling out of commodity prices" threatens the agricultural industry with bankruptcy.

Stenholm, like many others, said he was waiting for the president to tip his hand on where he wanted further budget cuts made. And, like many other conservatives, he said his constituents were ready to see the budget ax fall on the Pentagon as well as domestic programs.

"I certainly don't think defense should be immune," Stenholm said.

"Those of us who are national security-minded will have to look at some areas--not in a meat-ax way--but areas where spending might be delayed," said Mississippi's Montgomery, a member of the Armed Services Committee.

"People really hope the Reagan plan will work," said Rep. Charles (Chip) Pashayan Jr. (R-Calif.), a former Army captain. "They want to know where defense spending will be cut, but they're certainly not opposed to it."

The willingness of such conservatives to contemplate defense cuts may give Reagan vital bargaining room, for it was clear from their comments yesterday that moderate Republicans from the Northeast and Midwest who stayed in line on the earlier budget cuts are resistant to further domestic spending reductions.

Rep. Carl D. Pursell (R-Mich.), head of the so-called "gypsy moths," said he had advised Michel yesterday afternoon, after a meeting of his group, that any further reductions would have to be "across-the-board."

"A look at defense programs is long overdue," Pursell said. "If we get into defense, I think our group would be willing to negotiate. But if it's just domestic spending again, our view would be to hold the line."

That warning was underlined by Rep. Lawrence J. DeNardis (R-Conn.), who gave the "gypsy moths" their name. "Frankly," he said, "I've lost a bit of my confidence in the administration because of the 11th-hour concessions they made to the oil industry to get the tax bill passed. That was political expediency, and I'm not going to accept everything that comes from the White House as sacrosanct. I'll be a harder sell from now on, and I think most of the gypsy moths will be a harder sell."

Balancing that view is the general recognition that Reagan still enjoys broad support. "People are beginning to wonder about his program," said House Democratic Whip Thomas S. Foley (D-Wash.), "but Reagan is still popular." Some members of both parties--like House Republican Policy Committee Chairman Richard L. Cheney (R-Wyo.) and Budget Committee activist Leon Panetta (D-Calif.)--think the greater danger is that the White House could be pushed into ill-considered, hasty action.

But the congressional consensus appears to support the clear sense among Reagan's advisers that further steps must be taken quickly to make his budget cutting credible to the financial markets.

"Reagan is going to have to move decisively this fall to cut further spending," Rep. Newt Gingrich (R-Ga.), an ardent advocate of supply-side economics, said. "Most of my voters don't quite blame Reagan yet for the interest rates, but they say he better do something. He got all the praise, and he's the one who will hear the boos."

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said he has suggested to President Reagan that he begin talking to business community leaders in an effort to enlist their aid.

Baker said it was "incredible" and "absolutely appalling" that after the administration's and Congress' efforts in the last six months, financiers have not reacted.

"I hope and expect we will see a favorable reaction," he said. "I hope we get a fair shake."

Baker warned that he was talking not in terms of weeks or months, "but days."