WALTER F. MONDALE was seen lunching last week with Robert Strauss -- no, not in the wax museum, at a restaurant on Connecticut Avenue. Remember those two guys? Barely. Their politics are done with, probably -- hopefully -- forever. A new show will go on.

What kind? We could get tragedy, or comedy, or another heroic melodrama like Ronald Reagan's first term. It's too early for a prediction, but we do know enough to start thinking intelligently about what's coming.

Conditions have changed considerably just since the election. The economy is stumbling, with intimations of recession in the air; the stock market is nervously down; the business community is edgy about administration plans to close its tax loopholes. The economy provides the playing field for American politics, and the field suddenly looks rocky.

The political situation has changed, too. New Republican leadership in the Senate foreshadows a new kind of Senate-White House relationship. Deep fissures within the GOP are already beginning to appear as the magic of the Reagan landslide begins to wear off. The sudden prospect of a fight over the Democratic Senate leadership offers the prospect of something new and unexpected. Democrats generally seem to have shucked Walter F. Whatshisname faster than most people change out of their pajamas.

The "pragmatists" in the Reagan White House, meanwhile, have been busy trying to push Washington and the country away from the happy-days euphoria of the Reagan campaign toward a grim new view of the budget deficit. The president's promise that growth would take care of everything was buried in record time; now, we're told, every federal spending program from Head Start to health research must be sliced -- and many totally eliminated -- to right the budget in times of insufficient revenues.

So what might 1985 be like? Let's consider some of the relevant portents.

Republicans first. They have a landslide-winner at the helm, but they also have something Washington hasn't seen (except in the unrevealing special case of Richard M. Nixon) in a quarter century: a lame duck president.

What does it mean to be a chief executive who cannot seek reelection? The last relevant example was Dwight D. Eisenhower, whose second term prepared the ground for a Democratic victory in 1960. Ike didn't do badly in those four years; arguably he was more active than in his first term. But he had no dramatic program to enact, and his economic policy failed to prevent both a damaging recession in 1958 (which helped the Democrats make big gains in Congress that year) and more untimely economic troubles in 1960.

Ronald Reagan has a bigger task in front of him. His "revolution" is far from complete. To bring it to fruition he would have to win legislative victories in the next four years at least as impressive as those he won in 1981. But in the House Reagan's position is much weaker than in '81; and in the Senate, nervous Republicans who fear the Democrats could retain control in 1986 have already demonstrated their edginess by electing an independent new leadership that will follow Reagan's lead only as long as it serves Republican senators' political interests.

In 1981 Reagan's political strength intimidated members of Congress. The best evidence of that was the Democratic votes for the final Reagan tax and budget packages. In the Senate, Democrats voted for Reagan's budget cuts by 31-13, and for his tax cuts by 26-7. In the House, 29 Democratic votes put the budget cuts into law, and 48 Democratic votes provided the margin of victory for the tax cuts. Democrats in Congress are rightly wary of predictions that Reagan will lose his clout next year, but if you ask a dozen of them if Democrats could be intimidated again in '85 as they were in '81, they all say that would be difficult to imagine.

The emerging divisions within the GOP also bode ill for the president. Far right conservatives in the Senate were devastated by last month's leadership elections, when moderates and even Republican liberals (John H. Chafee of Rhode Island being the most dramatic example) won key posts, while the right- wingers were decimated.

The losers are bitter. Sen. James A. McClure (R-Idaho), the right's candidate for majority leader, won just eight votes and was eliminated after the first ballot. He proceeded to fire his close aide of a dozen years, Michael D. Hathaway, and went into a funk from which, colleagues say, he has not recovered. Sen. Jake Garn (R-Utah), defeated by Chafee for the post of chairman of the Republican Conference, was left "pale, tight-lipped and incommunicado," in the words of one Senate Republican aide. According to a reliable source, the far right has already convened one meeting, "attended by Jesse Helms (R-N.C.) personally," at which there was discussion of running right-wing opponents against sitting moderate Republican senators in 1986.

A similar situation may be developing in the House. The right wing there is divided between self-styled revolutionaries, typified by Rep. Newt Gingrich (R- Ga.), and others who don't think a severe New Right agenda is the key to future Republican success. One of them, Rep. Mickey Edwards (R-Okla.), wrote a veiled attack on the Gingrich group in last Sunday's Washington Post, and is now said to be organizing his own group of members.

Meanwhile, Reps. Olympia J. Snowe (R-Maine) and Thomas J. Tauke (R- Iowa) convened a meeting Thursday morning attended by 24 "centrist Republicans," in Tauke's phrase, to discuss organizing yet another Republican caucus. Tauke said 31 members have expressed interest in joining the group. "We believe strongly that there is a great need to have a Republican majority (in the House) . . . and if we are going to get it, we'll have to elect a lot of moderates," he said.

"A little friendly competition can strengthen the party" and can provide a home within it for a wider range of politicians, Tauke said.

The Republican stew was further spiced last week by the venerable Sen. Barry Goldwater (R-Ariz.), soon to become chairman of the Senate Armed Services Committee, who foreshadowed a big fight over military spending in a startling interview. Defying his president, Goldwater proposed junking the MX missile and freezing defense spending. As godfather to the right wing, Goldwater provided ideological cover for any Republican who would care to join him in those positions next year. He preemptively denied Reagan any possibility of making the MX or his mammoth defense increases into pure partisan issues.

So far, a senior Republican aide in the Senate observed, this "Balkanization of the Republican Party" has been papered over by Ronald Reagan's 59 percent victory last month. But this source foresees "very bitter internecine warfare" among Republicans in the months ahead.

Democrats are hardly positioned to take full advantage of a Balkanized GOP They are still reeling from November's results, although they take some comfort from the Republicans' failure to make significant gains in Congress.

Unexpectedly, the Senate Democrats will have a chance next week to demonstrate a new seriousness of purpose. Until late last week it appeared inevitable that Senate Democrats would reelect as their leader the ineffectual Robert S. Byrd (D-W.Va.), a man whose time arguably never came, but in any case has certainly passed. Then Sen. Lawton Chiles (D-Fla.) announced he would challenge Byrd for the leadership, providing an opportunity for both symbolic and substantive change that could help revive the Democratic donkey in a stroke.

The case against Byrd is powerful. In 1981, after Democrats lost control of the Senate, Byrd went into a tailspin. Members spoke of him as a dethroned prince, helpless without his power and status as majority leader. Byrd regained composure, but provided nary a whit of leadership for the Democrats.

Chiles is no Sir Lancelot, but his intelligence and quiet resolve could surely be an improvement over Byrd. More important than his own persona, though, is the symbolism of change his selection would provide. Just before Thanksgiving a senior Democratic senator said, "This leadership thing over here (in the Senate) is critical, and yet it is going to go unresolved."

Reached on Friday, the same senator was elated by Chiles' decision to challenge Byrd. However, he added, if Chiles' challenge is defeated, the result "will be worse than if nothing had happened." Byrd's reelection now would be confirmation that the Democrats in the Senate are incapable of exploiting a rare opportunity to follow the prescription of all of their political doctors and begin building a new Democratic Party around new personalities.

House Democrats may be similarly incompetent. Knowing that their beloved Speaker, Thomas P. (Tip) O'Neill, will retire in 1986, they made no move to replace him, but even disaffected younger members say they doubt that a new face will be chosen as O'Neill's successor. This may be a misreading of the mood among House Democrats, but no one could argue that a wind of change is blowing through the House Democratic contingent; occasional gusts are all that can be seen so far.

The Democrats' problems are on display in their current effort to select a new chairman of their national committee, a job that has symbolized Democratic difficulties for some time. Strauss held it in the post-McGovern years, when he steadfastly refused to put the Democrats into the computer age by matching Republican efforts to organize direct-mail fundraising. (As a result, the "fat-cat" Republicans became the party of small givers; the "common man's" Democrats became dependent on fat cats.) Charles Manatt did better in recent years, but his DNC was a hodge- podge of expensive offices run by representatives of party constituent groups who did little for the party's fortunes.

Now six candidates are running for the chairman's job, a contest for the votes of 377 members of the national committee. All six were in Illinois Friday wooing 11 votes. Several have set up elaborate Washington offices with computer tracking, press secretaries, parties for the press and more. (Next Thursday at Il Giardino, the Italian eatery, you'll find Duane Garrett of California wooing the political press over lunch.) Democratic governors are running about looking for a good new face who really is a Sir Lancelot to save the party, so far without any luck. Loyal Democrats say all of this maeuvering is inevitable, but what would Harry Truman say?

Particularly telling is a fund raising letter for one candidate for chairman, consultant Robert Keefe. "Since this is not an election for public office," wrote Keefe's finance chairman to potential donors, "there are no restrictions on individual or corporate contributions and no reporting requirements." Step right up and buy a piece of your chairman of the Democratic National Committee.

It seems premature to guess how the battles of 1985 will go. At the moment the White House seems determined to confirm that we "ain't seen nothin' yet" by unveiling a series of horrific budget cut proposals that seem intended to scare all the players, and probably will.

There's no disputing President Reagan's considerable advantages as the battles begin. He can claim a mandate, and we know from experience that he knows how to build support for his program. But his record includes more than successes; in the last two years his budgets got nowhere in Congress. A budget this year that proposes to wipe out much of the non-defense government in Washington could easily follow that example.

One wise Republican close to the Senate leadership predicts that there is "a 30 percent chance" that Republican senators can concoct an alternative to a Reagan budget combining some kind of budget freeze, significant defense cuts and some gimmicks -- "freezing" of tax indexing, perhaps, before it comes into effect next year -- that would raise revenue, and that Reagan might embrace such a scheme. But the plan would have to reduce the growth of entitlement programs, including Social Security, this source said. Reagan seems adamantly opposed to violating his campaign promise not to touch Social Security -- at least for now.

Given the vastness of the budget deficit, the political risks of big spending cuts and the multiplicity of predilections on Capitol Hill, a 30 percent chance isn't a bad bet. But it does mean the odds are better than 2-1 against a successful package deal. And what will happen if there's yet another budget deadlock in 1985?

Oh, baby. Wall Street will despair, with ominous consequences. The Republican Party could "implode," as one Senate aide put it. The Democrats' chances for regaining the Senate in 1986 could soar. And the man in the White House, who likes campaigning a lot more than governing, will be left to contemplate his place in history.

At that point, given the rules of the Washington game, he may turn his attention to foreign affairs.