Texas Gov. George W. Bush, fresh from victory in Iowa, acknowledged today that he faces a greater challenge in next Tuesday's New Hampshire primary but asserted that a victory here would seriously damage the viability of his chief rival, Arizona Sen. John McCain.

McCain, who skipped the Iowa caucuses and finished fifth, said he was ready for "the main event" here in New Hampshire, doubted that Iowa's results would affect voters here and expressed confidence about his chances of winning.

"A lot of time in New Hampshire, they don't pay a lot of attention to the Iowa caucuses," he said on ABC's "Good Morning America." "It was a low voter turnout. We are going to have a very big turnout here in New Hampshire."

Bush and McCain, who are locked in a competitive contest here, began the final week of campaigning in the state battling a winter snowstorm and a new wild card in the Feb. 1 primary in magazine publisher Steve Forbes, who hopes his second-place finish in Iowa will give him a boost here. Forbes today cast himself as the "independent outsider" in the GOP field.

With campaigning slowed by the weather today and likely to be overshadowed later in the week by President Clinton's State of the Union address on Thursday and the Super Bowl on Sunday night, the Republicans pointed to Wednesday night's nationally televised debate as a make-or-break opportunity to sway voters.

Most polls in New Hampshire give McCain an edge over Bush, although the margins vary from 5 percentage points to 12 percentage points. Forbes trails both by a wide margin.

Meanwhile, Utah Sen. Orrin G. Hatch prepared to quit the GOP race after his last-place finish in Iowa, while Alan Keyes, who ran third in the caucuses, aired television ads urging Republicans to vote their consciences and not the polls.

Bush credited McCain with running an excellent campaign in New Hampshire and said he could survive a loss to McCain on Tuesday. "I'm in it for the long run," he said in an afternoon interview. "I will be the last man standing."

Bush said he had gotten a slower start than McCain but argued that he has "a better message" for the voters here. A Bush victory on Tuesday, he said, "would obviously make it very difficult" for McCain to win the Republican nomination.

"If you base your strategy on one or two states and you can't win those one or two states . . . you're pretty well boxed in," he said.

Bush also took issue with a new McCain ad airing in New Hampshire that claims there is "only one man running for president who knows the military and understands the world."

"There may be some folks who resent the fact that John's claiming he's the only leader in the race," Bush said. "Of course I disagree with that. I believe I'm in a position to lead. That's what Iowans said yesterday and I believe folks in New Hampshire will say a week from today."

McCain today stuck to the strategy that has boosted him in New Hampshire, a virtually nonstop series of town meetings across the state. In a potentially ominous sign, McCain's campaign bus, the "Straight Talk Express," broke down at the height of the storm, forcing him and his aides to board a second, backup bus.

Even that did not seem to dampen the optimism of McCain and his campaign staff. "I think we've done everything we can do and we're comfortable with the plan," he told reporters.

McCain's theory--that he can begin to derail the juggernaut of the front-running Bush--now faces its first real test. Bush won the votes of 41 percent of Iowa's Republican caucus-goers, finishing ahead of Forbes, who had 30 percent of the vote, and Keyes, who captured 14 percent. McCain won 5 percent of the Iowa vote, running behind Gary Bauer's 9 percent.

Asked about Forbes's strong showing in Iowa, McCain said, "I don't care. The more the merrier."

McCain strategists said today that they doubt the wealthy magazine publisher will be able to turn himself a major force in the GOP race. "He has some appeal to Bush voters on taxes and the economy, and he has some appeal to our voters as the outsider independent," said McCain pollster Bill McInturff. "But our vote is strongly committed, and I don't think Forbes did well enough in Iowa to shake the Bush vote."

A Bush strategist agreed. "We'll get a little out of [Iowa]. He'll get even less."

But one weapon available to Forbes was on display here today: the conservative Manchester Union Leader, which has endorsed Forbes's candidacy. Today's front page carried three references to Forbes's second-place finish in Iowa, with the main headline reading, "Gore romps, Forbes tests Bush in Iowa."

Bush sought a preemptive strike against a potential negative ad campaign by Forbes, warning that voters would reject "ugly politics." But his campaign advisers say even if Forbes launches an attack against Bush, it may be too late for it to have much effect.

New Hampshire pollsters also said many voters here still resent Forbes for the negative advertising he ran against Robert J. Dole in the 1996 primary. "The big problem Forbes has is that 39 percent of likely Republican voters say they will never vote for him," said pollster Dick Bennett.

McCain was accompanied for part of the day by New Hampshire GOP Chairman Steve Duprey, who is neutral in the nomination battle. Referring to the Iowa results, Duprey said, "Bush is clearly going to be energized. Those are great numbers no matter how you slice it."

Staff writers David S. Broder and Ben White contributed to this report.

CAPTION: Sen. John McCain (R-Ariz.) greets schoolchildren in Sunapee, N.H., after an appearance at their elementary school.

CAPTION: Texas Gov. George W. Bush (R) during a TV interview upon his early morning arrival at the airport in Manchester, N.H., hours after Iowa caucus victory.