Sen. Lowell P. Weicker Jr. (R-Conn.) is in hot water with at least one Senate colleague for trying to walk both sides of the political street.

Weicker, facing a tough re-election fight, was cheered wildly last week when he condemned President Reagan and Republican conservatives before the annual convention of building trade unions here.

"I belong to the party of Lincoln and Eisenhower, not the party of Thurmond and Helms," Weicker said, referring to Sens. Strom Thurmond (R-S.C.) and Jesse Helms (R-N.C.).

While that may have sounded good to trade unionists, Senate Judiciary Committee Chairman Thurmond did not like it one bit. "He was amazed at that remark," a Thurmond aide said. "He didn't think it was necessary to make."

Thurmond, a longtime conservative, was especially bothered because he had allowed his name to be used in connection with a Weicker fund-raising event last November. Invitations to the $100-a-head reception said: "Please join us in a salute to Lowell Weicker, our friend from Connecticut."

Thurmond was not a sponsor of the event, but his name appeared on a list described as that of "members of Congress who will be present," which appeared in the program on the page opposite the names of sponsors.

Forty GOP senators, including such other staunch conservatives as Roger W. Jepsen of Iowa and Orrin G. Hatch of Utah, were on the same list. Helms' name did not appear.

One of Weicker's aides had called Thurmond to ask permission to use his name, according to a Thurmond spokesman. Thurmond, a good fund-raising draw as chairman of the Judiciary Committee, agreed as a favor to a fellow Republican, the spokesman said.

"They don't always agree on everything, but they have a good working relationship," the spokesman said.

For reasons unrelated to the flap, Thurmond did not attend the reception.

Weicker, who made a pointed effort to put distance between himself and GOP conservatives at the building trades convention, denies having asked Thurmond to help raise campaign funds.

Brendan Fitzsimons, Weicker's press secretary, said it would have been incorrect for anyone receiving an invitation to the fund-raiser to think that Thurmond was a sponsor. "You could paper the walls with invitations like this," he added.

Thurmond is more puzzled than angry about the incident. "He didn't particularly like what Weicker said about him," a spokesman said. "He didn't understand why he'd talk like that."