When Rep. Marc Lincoln Marks limped to the well of the House on Tuesday to deliver a speech, the House of Representatives was all but empty. The white-haired Republican from western Pennsylvania, whose aching back has forced him into retirement after three terms, had an audience of one Democratic congressman, three staff aides and the superintendent of the cloakroom.

But on the podium was the Speaker of the House, listening with rapt attention as Marks unfurled his "mea culpa" for voting last year for what he now calls "the murderous mandate" of Ronald Reagan.

Speaker Tip O'Neill did not stir during the half hour. What he was hearing was a script for Democrats campaigning this fall. Marks was the second Republican in a week who unloaded on a heretofore inviolable president, the second to say that Reagan is responsible for the misery in the country, that his policies are ruinous to the people, the party and Republicans seeking reelection.

Bob Packwood (Ore.), chairman of the Senate Republican campaign committee, had led off with an extraordinary interview in which he implicitly endorsed Clark Clifford's description of the president as "an amiable dunce." Packwood's observation that when he tried to talk deficits with Reagan, the president countered with a horror story about someone who bought vodka with food stamps, enraged the White House and delighted Democrats. A year ago, they were glumly contemplating 40 years of Republican rule.

"I remember," Marks said, "not too long ago that even our most conservative colleagues from this president on down stated publicly that the black community and the labor community had finally seen the light and could become a part of the Republican Party--the party of the people."

Now the Democrats are standing by in wonder as the Republican consensus unravels. They do not have to tell the president that his budget is dead and his policies doomed--Republicans do it for them. As Marks was speaking, Reagan was on the Senate side of the Hill, smiling and joshing and again saying he is open to budget changes, except, of course, on the only two items that matter, tax and defense cuts.

Marks says that until back surgery forced him to the bench, he seriously considered running as an anti-Reagan Republican in his largely Democratic Pennsylvania district. Such heresy is not abroad.

Rep. Jim Leach (R-Iowa), an outspoken moderate who agrees with Marks, found the speech too harsh. Many Democrats ran against Jimmy Carter in 1980, but financial ties that bind Republicans to their campaign committees are strong, and they have, Leach says, "a desperate desire to be friendly to the president if possible."

O'Neill, for obvious reasons, regards Marks as "a man of courage."

The Speaker is no less overweight, no more telegenic than he was a year ago, but the complaints of his flock are muted these days. The O'Neill strategy of putting Reaganomics to the earliest possible vote has been vindicated. What is unfolding now--high unemployment, the wails of the poor--is the president's doing.

Twenty-four hours after he had listened so raptly to Marks, the Speaker stepped out to join the Democrats' peace movement, the initiative for a nuclear freeze. O'Neill had his reservations. He fears a return of the longhaired, flag-burning lefties who infested the Vietnam protest but, in the end, he could not give the back of his hand to another issue so generously provided by Reagan's wild talk about nuclear warfare.

At an exceptionally high-powered, high-hearted meeting at American University, Sen. Edward M. Kennedy (D-Mass.), with Sen. Mark O. Hatfield (R-Ore.) as co-author, introduced a congressional resolution for a joint U.S.-Soviet halt on the manufacture and deployment of nuclear weapons. A panel of scientists, clergymen and educators of the caliber Kennedys can always rally spoke eloquently about the "arrogance" and "blasphemy" of presuming to bring an end to God's creation.

Hatfield, the Senate's most liberal Republican, told of seeing Hiroshima after the bombing and spoke of the "commitment for peace that comes from the hearts of our people."

Rabbi Alexander M. Schindler of the Union of American Hebrew Congregations raged that "there is nothing clean about a device that can put a torch to civilization."

Actually, Kennedy and fellow Democrats are merely stepping nimbly to lead an army already on the march. Rep. Edward J. Markey (D-Mass.), who has collected more than a hundred House cosponsors, noted the "groundswell at the grass roots." Randall Forsberg, a founder of the movement, reported freeze activity in 43 states.

It does seem like springtime to the Democrats, especially to the Speaker. Reagan has handed them two potent issues: the economy and peace. If they are not careful, they could win the congressional elections in the fall.