Consumed by Gorbachev-spawned euphoria, a beaming Ronald Reagan turned to a guest before his White House dinner Tuesday night and asked, with unconcealed pride, whether he had noticed that the stock market ''was up almost 50 points.''

That a president whose administration has been spooking the financial community with an uncertain economic trumpet the past two months was now crowing about a ''Gorbachev market'' fit the surrealism of summit week. The giddiness went beyond Nancy Reagan (''this is the Hollywood bash she always wanted,'' a friend said privately), infecting dominant figures at the White House and State Department. But it deeply depressed the president's old followers.

Absence of U.S. restraint makes this summit a turning point for the administration and the conservative movement Reagan has led for two decades. Many Reaganites, including the remnant still in senior posts, will never look at him quite the same. Sen. Steve Symms, one of only two non-California House members who backed Reagan for president in 1976, calls his performance ''an absolute disaster'' and the summit ''a love-in while they keep killing Afghans.'' Though less harsh, others see dangers in the way the White House transformed what should have been a businesslike encounter.

The danger was warned of last April by Richard Perle, then assistant secretary of defense. Predicting a year-end summit, he told us that it need not reenact Jimmy Carter's embrace of Leonid Brezhnev: ''I don't think you've got to have the hugging. I think a realistic summit, a summit that reflects the true state of the relationship between us will be a restrained summit . . . competition {with} a totalitarian state that means us harm'' will continue.

No such ''restraint'' was counseled during White House planning sessions or presidential briefings. Reagan's presummit interview set the tone. He excused Gorbachev for the rape of Afghanistan because he had not been in charge when Soviet troops invaded.

The president's comments during the summit proper ranged from hyperbole (''a date that will be inscribed in the history books'') to hilarity over his new friendship.

But the Soviets were dominant in determining the tone, led by a Gorbachev who engendered respect and concern for what one source called his ''manipulative skill and personal charm.''

Indeed, the president and his American friends were almost outsiders after the White House dinner when pianist Van Cliburn kissed and hugged Gorbachev. Cliburn then led in singing a Russian ballad while the Americans sat in their chairs looking foolish. Vice President George Bush finally broke it up, rising from his seat and turning to the president to rise with him.

Instead of dominating, Reagan and his men seemed more interested in boosting de'tente. At the state dinner, Perle sat next to Gorbachev, and former U.N. ambassador Jeane Kirkpatrick next to Reagan. That seemed to stamp the spirit of those two Reaganite hard-liners on the soft-line summit proceedings, while in fact they have grave misgivings.

Perhaps the tipoff of White House determination to make the summit a happy affair was Reagan's invitation to all 2,300 of his political appointees, plus their spouses, to Gorbachev's farewell ceremonies at the White House yesterday. No visiting leader of an American ally or a great democracy ever got such a sendoff by this administration. How to handle the invitation became an ideological litmus test.

The uniformed military, immune to Gorbachev's charm, is not worried. It feels the tide of euphoria will reverse itself before Reagan takes the dangerous steps toward strategic nuclear disarmament. Civilian national security experts are more concerned than the Pentagon brass about the administration's failure to heed Perle's admonition of restraint. Unlike the powerless and programmable Russian masses, populations of the Western democracies are not turned on and off easily.

Therein lies the despair of the Reaganites. Not only has their leader disappointed them on the core issue of American conservatism. In this unrestrained Reagan summit, the president has tacitly endorsed the central theme of arms controllers he fought so long: that the chasm between Moscow and Washington in values and intent means nothing if nuclear arsenals are mutually depleted.