Memo to Tip O'Neill and the Democrates:

To get some idea of just how well this president is doing, ask yourself one question: When was the last time anyone referred to Ronald Reagan as an ex-actor? Let's be honest. The president deserves praise. He has restored a welcome zest and joy to our political life and has effectively set the national agenda and dominated the national dialogue. He has expertly galvanized both public and political support for his economic recovery program.

But Democrats should stop hanging crepe. Immediately. It's entirely possible that the first week of May, which included House passage of the Reagan budget bill, may turn out to have been the political high point of the Reagan administration. Just consider a few of the Republican president's problems and the Democratic Party's opportunities.

The Reagan Core Constituency and What to Do About It. Remember the 1976 Republican presidential primaries? Sen. Jesse Helms remembers, and you can't blame him. By the time of the North Carolina primary, Reagan was reeling from losses to Jerry Ford in New Hampshire, Florida and Illinois. Jesse Helm's support was crucial to Ronald Reagan's 1976 comeback victory in North Carolina and to Reagan's being able to carry his challenge to the Kansas City convention.

For 15 years, Reagan's support and political sustenance came from the right wing of the Republican Party. For nearly four months now, they have submerged their own agendas to the president's economic plan. But what now? Will Ronald Reagan turn his back on those conservatives like Sen. Helms who stuck with him against Jerry Ford, or will he make their caused his as he promised during his presidential campaign?

And whatever he does to redeem those earlier pledges or to meet those non-negotiable demands, Reagan must realize that the New Right cannot be fully satisfied. That would be fatal to the New Right, which prides itself on taking no political prisoners. Direct-mail fund raising thrives on dissatisfaction, and the New Right is as hooked on direct mail as the rest of us are on oxygen.

Jimmy Carter: Ronald Reeagan's Unmentioned Problem. Jimmy Carter. You remember him: former governor of Georgia; Annapolis; zero-base budgeting. Well, there honestly has not been a lot of Carter talk hereabouts recently, which could mean some trouble ahead for Ronald Reagan. Carter left no footprints. Reagan will not be able to run against him anymore, because Reagan has so totally eclipsed his predecessor. The economy is now Reagan's economy, not Jimmy Carter's.

You will recall that in October 1976 President Ford's campaign was still buying broadcast time to play its terrific jingle, "I'm Feeling Good About America," which was actually an implicit slam at Richard Nixon, some two years after he had left office. Reagan, less than four months into his own administration, is deprived of any such ploy.

Foreign policy. With the passage of the budget bill, the administration must soon admit that a swollen defense budget is no substitute for a coherent foreign policy. The Reagan foreign policy thus far has consisted of the secretary of state's getting tough on murdered Maryknoll missionaries in El Salvador and the White House staff's getting tough in the press on the secretary of state. Almost the whole administration joined in lifting the grain embargo, which seemed to reward the Soviets for their restraint in not invading a sovereign nation.

Reagan's problem could turn out to be first-cousin of Carter's. Carter's foreign policy was essentially that urged by the left of the Democratic Party.For this, Carter got blame from the right, and no credit from the left. Reagan's problem is not that much different. Which position will prevail, for example, in the question of East-West trade -- profits or principles?

Democrats' Discovery. From the budget defeat, you, as Democrats, must now understand that, in the House and the Congress, you are a minority party. Numbers mean zilch with a genuine leader like Ronald Reagan persuading in the Oval Office. You can now relax and start behaving like an opposition party without the responsibility for governing. Politics, as the wise guys have told us, ain't beanbag. Politics also ain't college basketball. So you can stop acting like you're playing the game with four personal fouls -- timidly and defensively. Now is the time to look forward, for example, to September, when this conservative president will have to ask this conservative Congress to raise the public debt ceiling to $1 trillion. And he will have to ask for your Democratic votes to do it.