David Stockman, the second most accomplished actor in the White House, is famous for his interpretation of such disparate characters as John Anderson, Jimmy Carter and Walter F. Mondale during Ronald Reagan's practice presidential debates. Recently, however, he got his fattest part. In drawing up the new budget, he played Reagan.

What the director of the Office of Management and Budget did was to incorporate Reagan's wildest dreams into the budget projections, to put down on paper things that Reagan never dared say during the recent campaign. Such sacrosanct items as veterans' benefits, farm subsidies, student loans and Medicare payments were slashed for all to see. The list astounded this listless city.

The word went out that this was a little drama dreamed up by White House pragmatists. The scenario called for Reagan to read the document in dismay, to imagine the howls from Congress and then, with a heavy heart, to reject the recommendations and begin the unspeakable task of cutting defense and, yes, raising taxes.

Instead, he apparently saw Stockman's version of himself and declared that it was him to the life, the very thing. He is going for it.

Not only that, he is going to fight for it. If Congress balks, according to White House chief of staff James A. Baker III, Reagan is prepared to go to the country and campaign for the cuts.

There is some question of how docile Senate Republicans will be. Twenty-two of them face reelection in 1986, and they may expect Reagan to do for them what he did for himself recently -- that is, to soft pedal his determination to reduce the size of the government. Reagan was no slasher on the stump. Instead he was heard to brag about his administration's increases in food stamp spending, a program he vigorously fought for many years.

If the president loves the Stockman budget, he does not seem to care for another economic initiative: tax simplification. During the campaign, he made much pious mention about the need for tax reform, but his heart was not in it.

Treasury Secretary Donald T. Regan, who has served in the Reagan Cabinet from the beginning, has become somebody the president scarcely knows. He presented his proposals at a jammed news conference and made the astonishing declaration that it was a "personal" program and that he had no idea how the president felt about it.

The president felt cool. The proposed tax changes sound quite sensible, but they will not appeal to corporations, for which he has a special tenderness.

The only thing that the president likes about the changes, it seems, is that they would be "revenue neutral," a bureaucratic term meaning that they would net the government the same amount of tax money as current law.

For the ordinary taxpayer who is reduced to terror by the sight of a Form 1040, the greatest appeal of tax simplification may be the Treasury secretary's promise that the dreaded Internal Revenue Service will in time help as many as 12 million citizens to fill out returns.

But the future of the worthy and just cause of tax reform is bleak without presidential backing, which is not forthcoming.

Certain Democrats, like Sen. Bill Bradley (N.J.) and Rep. Richard A. Gephardt (Mo.), are pushing for reform, but their brethren, knowing how few votes there are in the issue, do not respond.

Most Democrats are gloomily wandering through the wreckage of their presidential hopes and wondering how they can outwit, or live with, the great base stealer in the White House for four more years. The trouble is that when they cooperate he takes the credit. When they oppose him, he goes running to the country to complain of their "obstructionism."

Their one comfort may be that they have heard no talk of repealing the 22nd Amendment to the Constitution, which makes it impossible for Reagan to seek a third term.

They know that the budget cuts do not faze Reagan, who will offer them as a sincere attempt to cut the deficit. They will affect groups he does not consider his core constituency -- the elderly, the poor, the young -- although they, like everyone else, voted massively for him on Nov. 6.

Farmers gave Reagan 92 percent of their vote. Yet a group of them in Nebraska petitioned him to take it easy on farm foreclosures and other miseries that have overtaken them during his tenure. The Democratic governor, Bob Kerrey pointed out that they were an almost verbatim repeat of the Democratic platform statement on farm policy.

But such contradictions were overwhelmingly accepted by the voters. Stockman obviously understands them very well, which is why he played Reagan to the hilt. He is the actor's actor.