President Reagan struggled today to regain his campaign stride after a debate that his strategists acknowledged probably boosted the political fortunes of Walter F. Mondale.

Sen. Paul Laxalt (R-Nev.), Reagan's general campaign chairman, and White House chief of staff James A. Baker III said the president's closing statement Sunday night was ineffective.

Reagan pollster Richard B. Wirthlin said the president's 18-percentage-point lead could slip to 12 or 13 points by week's end.

Privately, there was near-consensus among Reagan campaign advisers that Mondale had put the president on the defensive in their nationally televised encounter in Louisville. Some strategists said Reagan, often underestimated by his opponents in the past, may have underrated Mondale and been overconfident.

One adviser also conceded that Reagan may have appeared tired and showed his 73 years during the closing stretch of the 100-minute debate, which ran 10 minutes over schedule. But this was discounted by others in the White House, especially spokesman Larry Speakes, who left before the debate ended to brief reporters.

While there were no signs of panic in the Reagan campaign, his advisers acknowledged concern about the president's performance at the close of the debate and expressed worry that press accounts giving Mondale the victory would add to the damage.

Reagan tried to be upbeat. When reporters asked his assessment as he boarded Air Force One in Louisville, he replied, "You're supposed to make those decisions." As the questions persisted, Reagan said, "I'm smiling, I'm smiling" and gave the thumbs-up sign as he disappeared into the plane.

After he spoke today in Baltimore's "Little Italy" neighborhood at the dedication of a statue of Christopher Columbus, Reagan was asked whether he would prepare differently for his second debate with Mondale. "No, why?" he responded.

Nonetheless, the president stumbled over phrases and statistics several times earlier today at an open-air rally in Charlotte, N.C., where he was introduced by incumbent Republican Sen. Jesse Helms, who is in a close reelection contest with Democratic Gov. James B. Hunt Jr.

In praising North Carolina, the president was supposed to say, "You cure tobacco, and we cure economies." It came out, "You cure tobacco, and I'm trying to cure economics."

In Charlotte and Baltimore, Reagan tried to regain the offensive by jabbing at Mondale on taxes and the deficit issue. "Contrary to what you heard in the last 24 hours, I do have a plan," the president ad-libbed in his Baltimore speech.

Reagan's advisers, who mounted a sophisticated effort to limit damage from the debate, said they would not know for several days whether the event had cut Reagan's lead enough to make Mondale competitive. Baker said the debate "sharpened differences" and, in doing so, may have helped Reagan.

Reagan strategists grudgingly praised Mondale, who Laxalt said "overall . . . was more aggressive" than Reagan. Reagan-Bush campaign director Edward J. Rollins said Mondale "performed better than people thought he would," and Wirthlin said he was "surprised" that Mondale had said he liked Reagan.

"He looked more presidential than ever before," Wirthlin said of Mondale. "I think he helped himself."

In a Wirthlin poll of 976 registered voters, 41 percent thought Reagan won the debate and 38 percent picked Mondale. However, the polling began 15 minutes before the debate ended, and those who watched all of it gave Mondale a slight edge.

This result deepened the view in the Reagan camp that the president's finish had been especially weak. Wirthlin said that happened because Reagan was "diverted" into answering Mondale and did not make the closing remarks he had planned.

Speakes said the president did not rehearse his closing statement, adding, "He worked it out on his own."

When Wirthlin's sample of registered voters who gave Mondale a virtual standoff in the debate was asked its choice as president, Reagan won, 55 percent to 37 percent.

Wirthlin said this margin is likely to close as the party loyalty of disaffected Democrats is firmed up in the campaign's closing weeks. Such closure, he said, would have been likely had no debate been held.

Reagan, who promised during the debate to keep the nation moving forward if reelected, tried to make this theme a refrain today. In Baltimore, he recalled that he had debated independent candidate John B. Anderson four years ago and talked of a "new beginning."

"Well, as demonstrated in yesterday's debate, the central issue in this election is whether we're going to keep moving forward or go back," Reagan said. "America, like Baltimore, has made a new beginning, and we aren't going back."

In Charlotte, Reagan denounced school busing to achieve racial balance and Mondale's proposal to raise taxes, and then he defended his military budget increases as a precondition for arms control.

"We are making quiet progress toward arms control because we have not been misled by false talk and empty promises," Reagan said. "We have put America in a position of sufficient strength to achieve real and meaningful reductions in nuclear arms.

"And we did this knowing, as Theodore Roosevelt said, that the cry of the weakling counts for little in the move toward peace -- but the cause of the just man armed is potent."

Reagan was cheered today at rallies that featured the usual banners and balloons, and aides said he would not change campaign plans, which include trips to Michigan Wednesday and Ohio Friday.

But there were also sprinklings of anti-Reagan signs, a few of which referred to the debate. Leaving Baltimore, the presidential motorcade traveled a side street where a lone protester held a sign reading: "So long Teflon. Fritz sticks."