Ronald Reagan's triumphant return to action last night was a potent reminder of the central truth that has emerged from his absence.

The truth is that the presence of the president is crucial to the success of his proposals. Without Reagan, Reaganism falters. With Reagan, the administration, party and principles he represents are once more in the ascendancy.

Even some of those closest to Reagan were surprised at the difference made by his absence. Everyone knew that he was the best salesman for his programs, but the old belief that he is simply a spokesman who leaves the affairs of his administration to others has clung to Reagan like a cloak.

When Reagan was gone, for all the efficiency of the men around him, key decisions were put on hold. The internal organization of the White House waited. Most importantly, Reagan suffered the first Republican defections from his economic programs, when three conservatives on the Senate Budget Committee bolted on the important symbolic issue of future deficits.

While Reagan's popularity soared in response to the courage and grace he displayed after he was wounded March 30, the momentum for his most cherished programs slowed.

Characteristically, Reagan sought last night to turn that popularity to a practical purpose. He began by expressing his gratitude to the nation and those who protected him against the rain of bullets outside the Washington Hilton.

But he quickly turned to a most specific pitch for his own program, in the process describing the Democratic substitute as "an echo of the past rather than a benchmark for the future."

The setting in which Reagan made that appeal had been carefully planned. Some suggestions called for the president's comeback speech to be made from the Oval Office or the Lincoln Library, but Reagan decided instead to risk the wear-and-tear of striding down the aisles and shaking hands for several minutes because it would advertise, as one aide put it, that "Ronald Reagan is back."

"There's a certain amount of comfort to that for the nation, and to the extent that helps politically, that's all right, too," acknowledged speechwriter Ken Khachigian.

While all the public opinion surveys show that the president's popularity has soared since the shooting, many polls also show that Reagan is more popular than his programs.

There is nothing especially new in this -- it was often the case during Reagan's eight years as governor of California -- but it is a reminder, as Republican pollster Richard B. Wirthlin points out, that "The president is essential to what he is trying to accomplish."

Wirthlin's latest surveys, completed April 14, show Reagan with a 76 percent approval rating, compared with 17 percent who disapprove and 7 percent who have no opinion. This is a gain from the already high figures in the previous poll, taken March 25, where the figures were 70-20-10.

Other poll findings are similar. An AP-NBC News poll taken April 13-14 show approval-disapproval ratings of 77-75-8. A Washington Post-ABC News poll of April 20-22 had ratings of 73-19-8. And a Los Angeles Times poll of April 12-16 had the highest approval rating of all -- 83 percent.

All these polls show strong public support for Reagan's economic programs, but the programs remain less popular than the president. The Times poll, for instance, shows that 45 percent of Americans believe that a tax cut "would get America moving again" as Reagan contends, while 36 percent think it would increase inflation, and 19 percent are unsure.

The good news for Reagan is that this is a 12 percent gain in support for the tax cut. The negative news is that less than a majority still back the essentially radical "supply-side" idea that tax cuts will increase revenues.

So what Reagan was trying to do last night was transfer his personal popularity to his programs. Wirthlin says that an increase in the president's "hard-core" support may give him an even better opportunity to accomplish this than before the assassination attempt.

In a measurement long used by Wirthlin, a hard-core supporter is defined as someone who is asked to say something negative about a person and responds with something positive. A hard-core opponent is the opposite: one who is asked to say what he likes about Reagan and mentions something negative.

Hard-core support for Reagan in the last two Reagan surveys went up from 21 to 32 percent, while hard-core opposition decreased from 10 to 8. Put another way, the ratio of hard-core support to opposition increased from 2-1 to 4-1 after the shooting.

"What the president was trying to do last night was put the ball back in Congress' court, but in a cooperative rather than a confrontational way," Wirthlin said. "He was saying, in effect: 'I was elected in November. Mr. Congressman, people voted for a change. Not it's your turn.'"

Non-confrontational the speech may have been, but there was an unmistakable political warning in it for the 63 conservative Democratic congressmen who know that the president could transfer his current cooperative manner to a campaign against them in 1982 if they vote to sink Reagan's economic program.

Some of them cheered almost masochistically when Reagan denounced "the old and comfortable way" of making small changes, and ended with the warning: "I think this great and historic Congress knows that way is no longer acceptable."

The president is back, said this sentence in terms so clear that they could not escape the dullest member of Congress. Or as a practiced campaign huckster might have put it in a political commercial: don't mess with Ronald Reagan.