Presidential adviser Sen. Paul Laxalt (R-Nev.) said yesterday that President Reagan fared poorly in his debate last Sunday with Democratic nominee Walter F. Mondale because he had been "brutalized by a briefing process" that overloaded him with statistics.

Reagan, without conceding that he lost the debate, gave a more subdued version of what had happened, saying he had done "a lot of homework myself, probably too much of it, without sitting back and relaxing."

Laxalt's sharp-tongued criticism of the debate preparation brought into the open private soul-searching in the Reagan campaign since the debate.

The president's advisers, pressed to find an explanation for a performance they consider unusually ineffective, are trying to defend Reagan, 73, from the charge that he is showing his age while also trying to find a way to prepare him better for his second debate with Mondale Oct. 21.

Campaigning in Ohio yesterday, Mondale intensified his call for wayward Democrats to come home and criticized Reagan for "impersonating" President Harry S Truman. Details on Page A20.

Laxalt, the Reagan campaign chairman and longtime friend of the president, told a news conference that Reagan "had an off night . . . but it wasn't because of any physical or mental deficiency. He was brutalized by a briefing process that didn't make any sense."

Laxalt spoke to the president before the news conference. Afterward, he met privately with Reagan's senior staff and repeated many of his comments. Later in the day, in an interview from Philadelphia where he accompanied Vice President Bush, Laxalt said of the president's debate preparation, "It filled his head with so many facts and figures that he lost his spontaneity and his visionary concepts." He vowed that preparations would be different for Reagan's second debate with Mondale, scheduled for Oct. 21 in Kansas City.

"We're going to let Reagan be Reagan this time," Laxalt said. "He's done reasonably well with that over the years."

Deputy chief of staff Michael K. Deaver, without directly commenting on Laxalt's charges, agreed that the debate preparation would be different next time and predicted that Reagan would "do just fine" in his second encounter with his Democratic challenger.

Though Laxalt mentioned no names, his comments were seen by several administration and campaign officials as directed largely at White House chief of staff James A. Baker III, who negotiated arrangements for the two debates; at presidential assistant Richard G. Darman, who directed debate preparations, and at Office of Management and Budget Director David A. Stockman, who impersonated Mondale in the debate rehearsals.

They declined comment, although others involved in the preparation process pointed out that Laxalt had made none of his criticisms beforehand when they might have done Reagan some good.

One official said that Stockman was deliberately aggressive in his impersonation and that Darman provided Reagan with stacks of statistics because the presidential assistant was intent on avoiding factual errors.

Baker, briefing reporters in Louisville on the morning after the debate, said, in response to a question about whether it had been wise for Reagan to debate Mondale, that the president had "put to rest all these charges that he's somehow out of touch, that he's not accessible, that he lives in a show-business cocoon."

He also observed that Reagan had been prepared in almost the same way he was when he faced President Jimmy Carter four years ago, a debate that many consider one of the decisive factors in Reagan's victory.