Sen. Paul Laxalt (R-Nev.), general chairman of the Republican Party, said yesterday that "restless" supporters of President Reagan who want to begin raising money for a re-election bid have been instructed to "cool it" until Reagan decides whether to run.

"We've told them to cool it, that in our estimation we've got a candidacy, and only he can determine when he makes that decision and how he handles it," Laxalt told a breakfast meeting of reporters.

Laxalt said that, under federal election laws, Reagan would have to give his blessing to, or disavow, any fund-raising or organizing on his behalf--and he isn't ready to make that choice. The senator said he has laid down "ground rules" that GOP activists should refrain from both.

Laxalt said that he expects Reagan to run and that an organization could be put in place in 30 days once the decision is made.

"I don't see anything now that would militate against a candidacy--nothing," he added.

Asked if he shares the assessment of other Republicans that Sen. John Glenn (D-Ohio) would pose the most serious challenge to Reagan, Laxalt said, "Sometimes I think John Glenn may be the Nelson Rockefeller of the Democrat Party. It may be that John may be the strongest Democrat that can be fielded in a general election, but, like Nelson Rockefeller, he could never get the nomination."

Laxalt also said Reagan should fire those aides who, in talking anonymously with reporters, have impugned the abilities of other White House officials.

"If it can be established that they are doing numbers on each other, they should be fired," Laxalt said.

He offered a vigorous defense of Interior Secretary James G. Watt, and said Reagan would have a "conservative revolution" on his hands if he asked the controversial Cabinet member to leave the administration.