IN CHICAGO on the coldest day of the winter, the Republicans staged a unity tableau, with their battered leader, Ronald Reagan as the centerpiece.

Sen. Charles H. Percy, who is up for re- election provided the occasion for the show of togetherness. For his million-dollar fund-raiser, which he modestly called "A Salute to the Senate Leadership," he collared such diverse types and presidential critics as Storm Thurmond, Lowell Weicker, Bob Dole and Pete Domenici, all fans of his, although not necessarily of the president's.

Whether this pretty picture of loyalty and mutual self-interest will survive the unveiling of the president's budget is not clear.

But the sight of the president at the head table, flanked at either end by such dissenters

as Budget Chairman

Domenici and Finance

Chairman Dole, who

have been shouting at

him through the public

print, was diverting.

Do they now think

that they have the

president in preventive detention or protective custody? Were they sending a message that as long as he is reasonable, they will act as if his survival is as of much concern as their own.

A pre-dinner press conference indicated that they realize that detente must be sought. Domenici promised that the president's budget would be "acceptable," with "credible" defense cuts, and that progress on the deficit would be "dramatic."

Senate Whip Ted Stevens denied angrily that there is "discontent" with the president in the Republican Party.

Domenici offered no specifics about the president's budget. He offered as proof that the supply-sider in the White House is now housebroken the fact that he and others had been called down several times -- and while the president had never said he was changing his mind, "obviously, he didn't have us there to exchange views."

Senate Majority Leader Howard Baker was not present. He was sent home by his doctor with what was described as "walking pneumonia." The president's political ailment is ebbing leadership.

The acknowledgement that the magic days are over and compromise and accommodation -- even with Democrats -- was not without pain for the president. Being at the Percy dinner cost him further with the friends of his heart, the right wing of his party.

Three floors up in the Hilton, at the same hour, a young, poker-faced conservative congressman, Tom Corcoran, was having a dinner which was aimed at telling the president that he might oppose Percy in a 1984 Republican primary.

In attendance were such companions of Reagan's long march to the White House as Paul Weyrich, in person, and Rep. Jack Kemp, by videotape.

Corcoran professed to hope until the last minute that the president would at least come by and say hello, although White House operatives say they told him months ago that this would not be the case. Corocoran was making the point that the conservatives have made with increasing shrillness that Reagan does not know who his true friends are.

But Reagan was repaid for forswearing the more compatible company upstairs.

Percy got up and went bail for him on the vexatious question of his authenticity as an arms controller.

The earnest chairman of the Senate Foreign Relations Committee asserted that "negotiations on limitations are now underway."

He addressed his guest directly, "I am convinced that you are devoted and dedicated to finding a way to cut nuclear weapons by a third."

The dismissal of Arms Control director Eugene Rostow brought to a rolling boil suspicions about Reagan's sincerity as a peacemaker and set off the damaging "disarray" charges.

Thurmond was the only speaker who paid tribute to the president in the extravagant terms that a sitting president expects to hear when he visits another city.

"America is fortunate in President Reagan," bawled Thurmond in his hollow voice. "He is the finest president we have had in this country in the 28 years I have been in Washington."

"I have encouraged him to run," he said. There was no encouraging roll of applause from the audience.

The president did not seem to enjoy the evening with his new probation officers. He had a much better time in the afternoon at Providence St. Mel's High School, an all- black parochial high school, where his stories of faking a baseball broadcast went over much better than they ever did at those trying sessions where Republican senators nagged him about cutting the defense budget -- and then went out and told the press they had told him to do it.