It's the last thing his super-protective staff would have thought of. But it turns out that what turns on Ronald Reagan is a heckler.

His trainers and managers spend their lives trying to shield him from close encounters of any kind.

But after what happened yesterday at the White House, they may change their ways. Their man can handle himself in a scrap.

Reagan, in the East Room with a California candidate who wanted to use the White House as a public convenience, was the man who won New Hampshire when he said:

"I paid for this microphone, Mr. Green."

The president has been off his feed these last few weeks, showing signs of petulance and panic as he waits for the other shoe to drop -- unemployment figures for September are to be released tomorrow.

At his last news conference, his voice had a noticeable tremor, and he gave other evidence of pre-election nerves.

The polls are maddening.

The public still thinks he's a nice guy but regards his economics as the pits.

The Democrats handed him his head on several matters in the closing hours of Congress. Republican candidates, who six months ago would have killed to be photographed with him, began sending back insulting messages, the essence of which was that he could come into their districts to raise money but not, please, to mingle with the voters.

Last weekend brought the disquieting disclosure that he wanted to cast George Bush as Spiro Agnew. Bush declined the part.

So the president threw a little mud himself. Out in Ohio, where aspirants for state office were reluctant to appear with the author of the state's sizable unemployment rate and had to be dragged to the platform Monday, he lashed out at the nuclear freeze movement.

On the way, he had seen some demonstrators, and he called them the dupes of "some who want the weakening of America." He probably knew this from reading his favorite periodical, Reader's Digest. He had failed to note that Sen. Jeremiah Denton (R-Ala.), using the same material, incurred a storm of McCarthyism charges in the Senate last week.

A witless explanation thrown out by Reagan at a "photo opportunity" -- "I did not have any Americans in mind" -- did not spare him scathing editorials. Adding to the impression of siege were a couple of moves that looked like preemptive scapegoating. Republican National Committee Chairman Richard B. Richards was sacked. He had been insufficiently ideological, and he had also committed the crime some months ago of suggesting that Reagan might not run for a second term.

With a nudge from the White House, Sen. Richard G. Lugar (R-Ind.) tentatively offered himself as a replacement for Sen. Bob Packwood (R-Ore.) as chairman of the National Republican Senatorial Committee. The Reagan staff has been laying for Packwood since he confided to the press that the president rambled on about vodka-drinking food-stamp recipients during a discussion of unemployment. Lugar's timing jolted his colleagues. He is in a reelection campaign, and senators wondered if voters would be amused by his presumption. They were not.

"We have enough trouble," one groused. "We don't know whether we're in for a wash or a wipeout."

That was the way it was, all unease and uncertainty, when 57 congressional candidates filed into the East Room yesterday to get a lift from a president who might be a drag on their hopes.

Their host was midway through his recital of the sins of Jimmy Carter when a loud voice broke in. It came from a tall, bearded man in a pin-striped suit.

The Secret Service paled, and other guests gasped as the man in the pin stripes unfurled a tirade about tax increases, Taiwan and Tylenol.

Someone else stood up at the other end of the room and began to clap loudly.

The voice of the master of the house rode over the din: "I don't know who you two are, but you haven't said a word that is true yet."

The other, more housebroken candidates were shocked. Howard Nielson, a proper contender from Utah, who sat next to the heckler, tugged at his coat and tried to sit him down.

"It was not the biggest tax increase in history," replied Reagan stoutly. He may not always know his facts, but he knows he is not going to be pushed around under his own roof. The challenge seemed to steady him.

"I thought this was for Republican candidates," he said cuttingly, to applause.

The heckler resumed his cries about the Trilateral Commission.

"Shut up," said the leader of the western world.

He got a standing ovation.

Outside, Republican professionals were ecstatic at the glimpse of a fighting Irishman. While reporters swarmed over the lawn looking for the mole -- right-winger Gary Arnold, who trails incumbent Rep. Leon E. Panetta (D-Calif.) in the 16th District -- Guy Vander Jagt (R-Mich.), chairman of the National Republican Congressional Committee, raved about the president's "steam, fire, pep, commitment."

Now the White House knows what to do. Plant a Gary Arnold at every campaign stop, and watch Ronald Reagan bloom.