Rep. Paul Simon should be depressed. He is a liberal Democrat from a southern Illinois district where President Reagan carried 21 of 22 counties last year.

Since Congress convened in January, he has been on the losing side, voting against the president's budget and tax programs and sponsoring an unsuccessful amendment to require congressional approval of the MX missile.

Moreover, this soft-spoken former professor is one of 13 House members targeted for defeat next year by the National Conservative Political Action Committee (NCPAC), which is already spending $40,000 on radio ads attacking him in his district.

But Simon is far from discouraged. "I refuse to be bullied," he said. "Back home, people are starting to talk about how they're going to have to pay extra for school lunches. The Reagan program is gradually beginning to sink in. There are issues Democrats can unite around: low interest rates, Social Security, arms control. I think there'll be a strong shift toward the Democrats in 1982."

It may be the lulling effect of a summer vacation, but the mood among the liberal Democrats in the House, those who have lost the most and still have the most to lose, is curiously optimistic as they prepare to return for more bruising battles with the White House next month.

The Republican president, the Republican Senate and conservatives in their own party have weakened their effectiveness but the reports of their death, they contend a la Mark Twain, are greatly exaggerated.

The brutal fight over the tax bill, in which Democrats lost not only the battle but also bargained away their principles in an orgy of special-interest bidding against Republicans, has soured them on the strategy of compromise. "We acted like a corps of trained seals," said Rep. Patricia Schroeder (D-Colo.).

Looking toward the 1982 elections, when the two parties will struggle for control of the House, Democrats are determined to stake out a separate identity, hoping that Reagan's economic program will fall flat, dragging down Republican candidates in its wake.

House Republicans managed to win repeatedly this year in an alliance with conservative Democrats, but they acknowledge it won't be so easy starting in September, when the White House seeks deeper cuts in social programs, and issues such as the Clean Air Act, the Voting Rights Act, Social Security, abortion, tuition tax credits and foreign policy dominate the agenda.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), the target of some criticism among his followers, sent a cheery letter on Aug. 7 to the 176 House Democrats who opposed Reagan's budget cuts.

He enclosed red, white and blue "176" lapel pins, declaring, "Wear them with pride! . . . Many of our constituent groups are now waking up to the Gramm-Latta budget cuts. A lot more of them will wake up Oct. 1 -- when the cuts begin to take effect."

Politically speaking, some Democrats are even expressing relief that their alternative budget and tax packages, which would have given the president nearly everything he wanted, didn't pass.

"The monkey is off the Democrats' back," O'Neill has said. "The cuts, brutal as they are, are the Reagan cuts. The deficit is the Reagan deficit. Reagan has won a big victory, but the horse that runs fast doesn't always run long."

Walter Fauntroy, the District of Columbia's House delegate and a leader in the Congressional Black Caucus' unsuccessful effort to present a third alternative to the Republican and Democratic budget bills, said, "I'm optimistic. I've been around long enough to know that the pendulum swings. If we are persistent, tenacious and principled, it will swing back."

Rep. Morris K. Udall (D-Ariz.), the dean of House liberals, also takes the historical view. "In 1964, the Republicans were pronounced dead," he said. "Two years later, that dead party elected 66 new faces to the House and two years later elected Richard Nixon."

Udall predicted, "By this time next year, it will be very apparent that the Reagan tax program is a turkey. It will mean big deficits and high interest rates. Then the Democrats will make a modest pick-up of 10 to 20 seats" in the 1982 elections.

Along with Wisconsin Democrats David R. Obey and Henry S. Reuss, Udall led a last-minute effort to enact a more modest tax bill, which would, he said, have avoided billion-dollar breaks to the oil industry and resulted in a balanced budget.

The liberal alternative received little attention amidst the duel between the Reagan bill and the official Democratic bill, shepherded by Ways and Means Chairman Dan Rostenkowski (D-Ill.), so it was a surprise when 144 members, a majority of House Democrats, supported it.

That vote, while little heralded in the fanfare surrounding the president's victory, marked the turning point away from what many Democrats call the "me-too" strategy of the spring and summer. "We got 144 and we didn't whore around town buying votes," said George Miller (D-Calif.), referring to the courting of oil-state Democrats by Reagan and Rostenkowski.

"There was never any question that the president would get all of what he wanted on the budget and tax bills, but the Democratic leadership adopted a policy of winning at any cost -- and the cost has been tremendous," Miller said. "In chasing around 29 southern Democrats, we lost any identification with what our party really stands for."

Rep. Thomas Downey (D-N.Y.), a member of Ways and Means, regrets having supported Rostenkowski's effort to outbid the administration for conservative votes. "We made a mistake," he said, in trading a balanced budget alternative for larger tax cuts and special-interest breaks.

"We mirrored the Republicans and we ended up with no platform to stand on. We suffered a major defeat, not only tactically, but emotionally and politically."

Nonetheless, Downey added, "I'm not discouraged. As Democrats, that leaves us with the recognition that our image has to change."

Although there have been isolated calls for O'Neill's resignation and widespread grumbling about the leadership's strategy, few Democrats are disposed toward having a bloody intraparty fight. Instead, the focus beginning this fall is likely to be on what political platform they can unite around.

"We've confused the hell out of our constituents," said Obey. "We've got to quit playing the inside game. We've got to build public understanding of what the real differences are between Republicans and Democrats. And I don't mean serving up the New Deal."

Social Security, high interest rates, how to stimulate productivity and cut waste in the Pentagon budget are only the most obvious issues, he said. "We have to stop talking about how much defense and instead what the defense budget ought to do."

Barney Frank, the outspoken Massachusetts freshman who arrived last winter boasting of being "a big spender," now thinks Democrats could do well to pin the big spender label on the GOP. "Why are we called big spenders for day care and legal services, but not for the Tenn-Tom a $2 billion administration-supported water project , the Clinch River Breeder nuclear reactor, or for peanut, tobacco, sugar price supports?" he asks.

The same arguments are made by Schroeder, who tried unsuccessfully this year to cut out administration-backed subsidies for the tourism industry, by Rep. Richard Ottinger (D-N.Y.), who tried to hold down the White House staff budget to 1981 levels, and by Miller, who has fought subsidized irrigation water for agribusiness, bargain rates on federal leases for the offshore oil industry and federal rather than corporate-funded workmen's compensation programs.

Back in his district over the recess, Miller attended several town meetings in Republican neighborhoods. "I thought I'd be put on the grill for my opposition to the budget and tax cuts. But people understand that the government is going to have to borrow money to cut taxes. They'd rather have a balanced budget.

"And I was amazed at the questions I got on the arms race. The elderly especially understand that they can't cut enough social programs to keep that military machine going at the rate they want to."

So, while Republicans may be in the saddle, and conservative, southern Democrats may be the media stars of the day, the liberals who have traditionally made up about half the House Democrats and have formed a vocal wing of the party seem to be down, but not out.

"More and more, we're electing people to Congress who hold their finger up to the wind to find out what's popular today," said Simon, who spent part of the recess working on a book about the future of the Democratic Party.

"The danger for Democrats is that we become a carbon copy of Republicans. We need more fiscal prudence, but if we lose our humanitarianism, we lose our soul."