The full field of Republican presidential candidates met here tonight for the first nationally televised debate of the campaign.

The forum in the studio of WMUR-TV, carried nationally by Fox News Network, gave the five challengers to Texas Gov. George W. Bush--who have sparred among themselves several times already--their long-awaited opportunity to test their arguments against the man who has led the popularity polls almost all year long. Bush skipped two previous debates here in New Hampshire and one in Arizona, where most or all of the others participated.

His absence from two previous forums in this state, where the first primary will be held on Feb. 1, drew widespread criticism in the New Hampshire press. Many observers say Bush's absence helped open the door for Arizona Sen. John McCain to make himself a credible upset threat in the contest, where McCain has focused his campaigning.

A Mason-Dixon poll of New Hampshire voters taken Nov. 29-30 and published today in the Concord Monitor showed Bush leading McCain, 41 to 36 percent, within the survey's margin of error. Publisher Steve Forbes was a distant third, with 7 percent, followed by talk show host Alan Keyes with 3 percent, conservative activist Gary Bauer with 2 percent and Utah Sen. Orrin G. Hatch with 1 percent. On the Democratic side, Vice President Gore and former New Jersey senator Bill Bradley were in a statistical tie, with 45 percent and 43 percent, respectively.

For Forbes, who earlier today began running an ad criticizing Bush's stance on Social Security, the debate offered an opportunity to jump-start an effort that Republican officials say so far has failed to generate as much interest as the New Jersey millionaire's 1996 campaign.

But Forbes will get one boost in this state Friday morning: The Manchester Union Leader will give him its endorsement. While "some would say he looks like a geek," the paper said, Forbes is "one tough, smart customer" who "can be a strong, principled leader."

Bauer, Keyes and Hatch all are struggling to make themselves well-known to New Hampshire's notoriously choosy voters, and the interest in Bush's performance probably guaranteed them the biggest audiences of the campaign.

Bush entered the forum as a relatively untested performer, having debated only once in each of his two successful Texas gubernatorial races. Texas reporters who accompanied the governor here said he survived those debates with two different Democratic candidates but certainly did not dominate them.

But Bush took a number of steps to bolster his position coming into tonight's contest. On Nov. 19, he delivered a major address on foreign policy, seeking to shore up his credentials in an area where he has had little firsthand experience. Bush's failure to identify more than one of the four leaders of nations in the news he was asked to name by a Boston TV interviewer earlier this fall had subjected him to some ridicule.

Two days ago, he set forth a detailed outline of a tax cut larger in scale than the Republican proposal vetoed by President Clinton late in the summer. Taxes are always an important issue in New Hampshire, and aides said Bush wanted voters here to know he intended to reduce the burden of federal levies, because most of his opponents already had laid out tax-reduction plans.

Forbes, who has carried over his 1996 campaign proposal for abolishing the Internal Revenue Code in favor of a flat tax eliminating most deductions, dismissed Bush's proposal as mere tinkering. Even before they stepped on stage for tonight's debate, Forbes said the Bush proposal was "politics as usual" and "a signal that . . . the gravy train is going to remain on track" for those who benefit from loopholes in the current code.

McCain, whose surge in New Hampshire polls has given the first hint that Bush could face a serious challenge for the nomination, got in some personal campaigning on the Seacoast before settling in this afternoon to prepare for the debate.

In well-attended town meetings Wednesday night in Exeter and this morning in North Hampton, McCain argued that campaign finance reform of the type he has sponsored without success in the Senate is a necessary precursor to overhaul of the tax system.

Pressed this morning by a questioner who asked, "Why not just abolish the code?" McCain said, "I would like to do that," but quickly added that it would be impossible "until you remove the special interest influence" of those who contribute large sums to the political parties.