Of all the parts Ronald Reagan has played in his long career in show business and government, his favorite role may be that of the citizen-politician. Through almost two decades, in which he has run for governor of California twice and president of the United States three times, and served in those two offices for more than 100 years, he has never dropped the guise of the average citizen, baffled and angered by what that cussed government is doing.

It worked in Caifornia and it has worked so far in Washington. But now Reagan is playing for bigger stakes -- both substantively and politically -- than ever before. His acting skill is going to be tested to the limits.

In denouncing both the process and the product of the House and Senate efforts to write a budget, Reagan has resolutely disowned responsibility for the unprecedented $200-billion-a-year deficits that loom ahead as far as the eye can see. He has made it clear that he will follow through on those denuciations by vetoing specifc revenue and appropriations bills if Congress, as expected, insists on attacking those deficits by trimming back both Reagan's tax cuts and his defense spending.

The risks for Reagan personally are as great as they are for the country. Already he is straining his credibility to the limits. He has said repeatedly that the sources of the deficits is runaway domestic spending, not be combination of tax cuts and defense increases. That is false.

The nonpartisan Congressional Budget Office analysis used during last week's Senate budget debate shown this: in fiscal years 1982 through 1984, congressional policy actions have added $43 billion to the deficit as a reault of defense increases; they have added $199 billion through tax reduction. And they have reduced the deficits by $145 billion by non-defense cuts.

In his televised press conference last week, Reagan told the nation that the budget he had submitted for fiscal 1984 "was the 1983 budget, the present budget, plus 4 percent across the board for domestic spending." Since "we have inflation down to less than 4 percent . . . if we had adopted that budget we would be giving a real increase, over and above inflation, of the '83 spending," Reagan said, "and I don't think we've done badly in '83."

That description is false. As the nonpartisan National Journal described the Reagan budget in February, it "proposes to hold spending growth to 5.4 percent -- a level equal to the expected rate of inflation," but not by an across-the-board freeze. "Military outlays would increase by $30.5 billion over 1983, a 14.2 percent gain . . . The remainder of the budget loss ground not only to inflation but in absolute terms as well. Spending for non-defense and non-interest purposes would be frozen in the aggregate, dropping from $501.5 billion in 1983 to $500 billion in 1984."

Those facts are known to members of Congress, and the continuation of that tilt has been rejected by majorities in both the Democratic House and the Republican Senate.

If the president persists in playing the injured innocent in the budget battle and distorting what Congress has done, he can only find himself in increasing conflict with members of his own party in Congress.

Twenty-one Republican senators joined with 29 Democrats-to-pass the budget that Reagan has denounced. Among those 21 senators were nine of the 18 Republican senators who likely will be on the ballot with himif he runs for reelection in 1984.

That raises some interesting questions about how far Reagan is prepared to go with his argument that everyone is out of step but Ronnie. Will he go to Mississippi and tell the voters that their senator, Thad Cochran, has joined the wild spenders in Congress? Will he make that argument to the voters of Wyoming about Alan Simpson? Both of those conservatives voted for the budget Reagan rejects.

Will he go to Oregon and tell the voters that Senate Appropriations Committee Chairman Mark Hatfield, who balanced his budget every year of the eight years he was governor; has lost his sense of fiscal responsibility because he voted to curb the Reagan deficits?

Will he tell the voters of Kansas that Alf Landon's daughter, Sen. Nancy Kassebaum, a member of the Budget Committee and suporter of the budget that has become Reagan's favorite rhetorical target, is unsound in her judgments?

Will he go to New Mexico and say that Sen. Pete Domenici, the chairman of the Budget Committee and the man who cast the deciding vote for the budget last week, is wrong when he says the congressional budget process is the best hope for fiscal sanity?

The question, in short, is whether Reagan will pursue this reckless strategy no matter what it costs his party, his country and his own credibility.