The largest crowd of the Democratic campaign jammed the statehouse grounds here today to welcome Rep. Geraldine A. Ferraro (D-N.Y.) as the victor in Thursday's vice-presidential debate and to hear Walter F. Mondale demand an apology from Vice President Bush for an "unpardonable" statement on the deaths of American servicemen in Lebanon.

The remark to which Mondale was referring occurred during a discussion of the Reagan administration's policies in Lebanon, where 267 servicemen have been killed in three terrorist attacks. Mondale and Ferraro have repeatedly criticized the administration's efforts to safeguard the Marines.

In Thursday night's debate, Bush said, "For somebody to suggest, as our opponents have, that these men died in shame -- they had better not tell the parents of those young Marines."

Mondale drew strong applause from the crowd of more than 25,000 who gathered under gray skies in this college town when he said, "We all make mistakes in campaigns, but that one is unpardonable. Mr. Bush, we love this country as much as you do."

"More," shouted many in the crowd.

"Mr. Bush, we honor our men and women who die in the service of our country, and we grieve for their families as much as you do. Mr. Bush, apologize, today, for that remark."

Mondale told reporters later that Bush's assertion was "an appalling and atrocious misstatement of the facts." He said that if anyone said what Bush claimed Mondale had said, that person "shouldn't be a dogcatcher, let alone president of the United States."

At a news conference, Bush said he would not apologize for the remark and refused requests from reporters to supply evidence that Mondale or Ferraro had said the men died in shame. "I know exactly what I was talking about, and I'll leave it right there," he said and walked out of the conference.

When told that Bush had refused to apologize, Mondale said, "He ought to be ashamed of himself. And if he thinks this is the end of it, he hasn't seen anything yet."

Ferraro was ebullient today after her face-off with Bush, telling Mondale, who was judged the winner of Sunday's presidential debate, "You know, Fritz, I think you and I would make a great tag team . . . . Now you're back in the ring, and they're all yours." Mondale and Reagan meet for their second debate Oct. 21 in Kansas City.

Nevertheless, Ferraro and her staff were pressed repeatedly to explain why, at the debate, she had reined in her usually exuberant speaking style in favor of a more measured and at times ponderous approach.

"I can be funny anytime. I can be flippant. I did exactly what I wanted to do -- talk to the American people," Ferraro told reporters on her campaign plane en route to Madison.

"If I were to do that debate tonight, I would do that exactly the same way. I am pleased with it. I was more interested that the American people get to know me," she said.

Ferraro's campaign manager, John Sasso, added later, "People know she's a fighter; people know she has strength. People know she's tough. We didn't have to emphasize that."

Ferraro said she also felt relieved and told Mondale after the debate, "I don't have to do that for another four years, do I?"

At this afternoon's rally, Ferraro returned to her customary spicy political style.

Noting that all four candidates had now debated, Ferraro proclaimed, "I beat George Bush and George Bush beat Ronald Reagan."

She also took a pop at the complaint by Sen. Paul Laxalt (R-Nev.), general chairman of the Republican campaign, that Reagan had done poorly in the debate because aides had "brutalized" the president and "smothered him with facts."

"Now I ask you," Ferraro queried, "since when is it considered cruel and unusual punishment to expect a president to learn the facts that he needs to govern?"

In an allusion to her printed rebuke of Bush Thursday night, Ferraro today charged that women athletes receive little more support from the administration than "a patronizing pat, which seems to be the Republican style."

Today's rally marked the sixth joint appearance of the two Democrats since Labor Day, excluding party fund-raisers, and the fourth by one or both candidates in this pivotal industrial and agricultural state.

Mondale continued to press his appeal for Democrats not to be lured into the Republican fold by Reagan's touting of the traditions of past Democratic presidents, including Harry S Truman. Reagan today began a whistle-stop train ride through Ohio in the same railroad car used by Truman.

"Today, Mr. Reagan is putting on a stunt that will be called and remembered as the great train robbery," Mondale said. "Harry Truman had his eye on the future and not on the past."

Mondale then quoted one of Truman's remarks during his famous 1948 cross-country train tour, when he referred to a small wooden toy known as the "floogie bird" that had a tag around its neck reading, 'I fly backwards. I don't care where I'm going. I just want to see where I've been.' "

Reagan and "the floogie bird party should listen to what Truman said at the end of that whistle-stop campaign," Mondale said. "The Republicans have the propaganda and the money. But we have the people and the people have the votes, and that's why we're going to win."

Before leaving Philadelphia, two Democratic congresswomen were paraded before the media to offer their views on the debate. Rep. Barbara A. Mikulski (D-Md.) said, "I don't know who won or lost the debate, but I think George Bush lost his marbles a couple times."

Asked whether Bush was disrespectful in addressing Ferraro as "Ms." or "Miss" rather than "Congresswoman," Rep. Barbara B. Kennelly (D-Conn.) quipped, "It's better than what they've been calling her."