RONALD REAGAN's early hero, Franklin Roosevelt, saved capitalism and made the nation safe for the Republican Party, paradoxically by using the tools of big government. Now, Republican President Reagan, using methods they abhor, may turn out to be the savior of Democratic liberalism.

Roosevelt, a marvelous communicator who could take his case to the country on the then-modern medium of radio, did for his rivals what they could not do for themselves. Most Republicans were simply too committed to unregulated capitalism and the balanced federal budget to accept the variety of emergency measures and social welfare experiements which became the New Deal.

And most present-day Democratic politicians, at least in their public incarnations, are too committed to once life-saving and now runaway social programs to impose the controls needed to save the program, their party, and possibly the economy which fuels their largesse.

The more perceptive Democrats can see this, too. As one Democratic official of long acquaintance declares: "From a political point of view, the best thing that could happen would be for the budget cuts to pass with the Democrats voting against them."

This is a private view. Publicly, this official derides many of the proposed reductions with politically suitable references to Republican heartlessness or Reagan simple-mindedness.

But this official knows that a number of beloved New Deal/Great Society programs, which for the most part came into being over the votes, rhetoric and dead bodies of Republicans, are , as Reagan loves to say, "out of control." And he knows, too, as do many Democrats, that it is next to impossible for the Democrats to cut back these programs on their own.

A description of what is possible in politics is usually a description of what politicians can take credit for achieving. Democrats are both benefited and burdened by constituency group politics, and the burdens are greatest when it comes to budget cutting. Typically, if a Democratic president, such as Jimmy Carter, or a Democratic governor, such as California's Jerry Brown, trims a social program, he will earn the opprobrium of Democrats for trampling on sacred soil and the criticism of Republicans for not violating it enough. A Republican president, particularly Reagan, has more freedom. Admittedly, because of the pressures of lobbyists, the divisions among Democrats and the exigencies of a process which in recent years has been controllable neither by presidents nor by congressional leaders, the result may be that the Reagan budget cuts do not in fact rein in the most expensive social programs enough to make a difference.

But if they do, or if Reagan is able to parlay this year's successes into future victories, both Democratic programs and Democratic politicians stand to be the winners.

Consider California, where Reagan in his second term as governor gained approval of a much-ballyhooed welfare bill which both reduced fraud and increased the grants of needy recipients. The bill was in fact worked out with the connivance and encouragement of the Democratic leadership in the state assembly. If Reagan took excessive credit for this achievement in two subsequent presidential campaigns, he was matched in this political display by Democratic legislators who successfully cast themselves as the real reformers of California's supposed "welfare mess."

The Reagan era in California, whatever else it did, was beneficial for the Democratic Party. The Democrats held a narrow margin in the legislature when Reagan came to Sacramento. They had a huge margin when he left, and they gained again last November in the teeth of Reagan's homestate presidential landslide.

What was true in California is even truer in the federal government. Rep. Leon Panetta of California, a compassionate, thoughtful Democrat who was a compassionate, thoughtful Republican until the policies of Richard Nixon proved too much for him, believes that it is in the interest of Democrats to reform programs like food stamps, providing that "a meat-ax" isn't used.

"We carry a large burden when food stamps are provided to students or to people above the poverty level," says Panetta. "It guts support for the program itself and plays into the hands of those who want to do away; with it."

Panetta believes that the "revolution" Reagan is promoting offers an opportunity for Democrats to reform and improve social programs which are necessary but which are justly criticized for inefficiency.

"I've told many disgruntled Democrats and special interst groups that if you look at the positive side of this, we can go after the waste which is part of the structure and better serve the programs and, quite frankly, the needs of the Democratic constituents."

Panetta compares the opportunity for the Democrats to that which confronted President Nixon when he decided that the time was ripe to recognize the People's Republic of China. A Democratic president who did what Nixon did would have been torn apart by the Republican opposition, probably with Nixon leading the way.

The Reagan revolution presents an even better political opportunity for the Democrats, if they do not cast themselves as obstructionists and allow Reagan -- a communicator in the Roosevelt model -- to blame them for whatever goes wrong. A more subdued Democratic approach will allow the Roosevelt party to once more cast its political appeal in terms of major social programs benefiting millions of people, while subsequently blaming Reagan for whatever goes wrong.

If Reagan does succeed in doing for Democratic liberalism what Roosevelt did for capitalism, he won't be thanked by the beneficiaries. The more FDR succeeded politically the more he was hated and reviled by what he called the "economic royalists." And the better Reagan performed in California, where he was a far more successful governor in his second term than in his first, the more calumny and criticism he received.

Nonetheless, he left behind a political climate where Democrats are more secure than when he came. The same thing could happen in Washington.