What people call ''Ronald Reagan's luck'' has been amply demonstrated again this past week. And once again, as so often in the past, it is the character of his adversaries that enables the president to look so good.

The latest volunteers include Sen. Joseph R. Biden Jr. and the top Democrats on the Senate Judiciary Committee, the television networks' White House correspondents and the right-wingers in his own party. The president did his own part, but playing off people like that, he didn't have to be terrific to look terrific.

That's the way it's been throughout Reagan's political career. When he first ran for governor of California in 1966, he came up against Edmund G. (Pat) Brown, the two-term Democratic incumbent. Brown had enough charm and evident good will to have survived a number of verbal and political gaffes. But after eight years, his administration had run out of gas and the voters had exhausted their patience. Enter ''citizen-politician'' Reagan, triumphant.

In 1970, the Democrats chose Assembly Speaker Jesse Unruh to challenge Reagan's bid for reelection. Unruh, a brilliant politician of uncontrolled temperament, had slimmed down from the 300-pound ''Big Daddy'' he had been, but the image of the roughneck boss of the legislature was indelible. Reagan won easily.

Ten years later, as the Republican presidential nominee, Reagan faced a Jimmy Carter battered by inflation, hostage-taking in Iran and repeated demonstrations of executive uncertainty. Result: a landslide. In 1984, he had Walter Mondale, carrying the burden of the Carter record and a personal antipathy to television campaigning. The Reagan landslide was even larger.

On his own, these last three years have been far from easy for Reagan. But once again in recent days he has benefited from the shortcomings of those he faced.

Consider the scene in the White House briefing room last Friday. The president of the United States has just announced an ''agreement in principle'' with the Soviet Union on the first arms-control treaty of the 1980s.

He pauses at the podium to answer questions. What does the television audience see and hear? Three grown men, certainly among the most prominent and presumably professional journalists in the land, start screaming separate questions -- "What about the shooting? What about the evil empire?" -- and none will stop long enough to let Reagan answer.

Some scene. Some journalism. No wonder the wire-service reporters visible in the TV picture hung their heads. I hold no brief for the White House news-management techniques: the habit of parading Reagan out to read from his script on such occasions and indicating he will leave before reporters have reasonable time to question him.

But when the TV reporters the public knows best act like five-year-olds, cranky because they missed their naps, it makes it all too easy for those watching to conclude that the president is well justified to avoid such encounters.

And what of the agreement the president announced? It is a modest achievement, affecting about 3 percent of the nuclear weapons in the U.S. and Soviet arsenals. But it is an achievement, and it opens the way for accelerated negotiations on the far more difficult and vital question of long-range nuclear missiles.

It is also a political gift from Reagan to the Republican Party. It shows the skeptics that building up U.S. military power is not simply an end in itself, but may make the Soviets more tractable. It gives the administration a diplomatic victory perhaps large enough to eclipse -- if not erase -- the damage of the Iran-contra affair.

The major GOP candidates for the 1988 nomination -- George Bush and Bob Dole -- recognize this. But those scrambling for a foothold in the right wing of that party -- Jack Kemp, Pete du Pont, Al Haig and Pat Robertson -- have decided to describe the proposed agreement as a threat to the West. Lacking the courage to challenge Reagan directly, they blame it on George Shultz.

With critics like these, Reagan looks magnificently moderate, a veritable statesman.

And just to make the symmetry perfect, the liberal Democrats on the Senate Judiciary Committee did a similar job on themselves while attempting to derail the Supreme Court nomination of Judge Robert H. Bork. In the crucial week of Bork's appearance, Biden's plagiarism problem drew almost as much attention as Bork's testimony -- and far more derision. If you look at the other senior Democrats on that committee, Edward Kennedy, Robert Byrd and Howard Metzenbaum, and reflect on the well-publicized incidents in their past academic, personal, political or business lives, you have to wonder who is in a position to cast the first stone. What the committee lacked was a man of Sen. George Mitchell's stature.

The senator from Maine, the conscience of the Senate's Iran-contra hearings, a former federal judge intellectually competent and politically prepared to go toe-to-toe with Bork, is not a member of the Judiciary Committee.

Just Reagan's luck. So what else is new?