So now there is a front-runner. By virtue of mathematics (the money he has raised) and chemistry (the way his rivals treat him), former Gov. Mitt Romney of Massachusetts is the leading candidate for the Republican presidential nomination.

There’s danger in that position. The GOP field didn’t gang up on Romney in their New Hampshire debate last week, but that’s only because many of the candidates are just getting their campaign legs and are breathing the sweet air of being in presidential politics for the first time.

Once they feel their political survival is at stake, their restraint will disappear, and one after another they will send arrows Romney’s way. Those arrows will provide opportunities for fellow contenders — and will challenge Romney’s mettle, providing evidence that he is strong enough to engage President Barack Obama in the general election or raising questions about his suitability as the party nominee.

Sometimes front-runners stumble. Democrat Gary Hart did in 1988, for example, but Hart’s fall was more a matter of personal conduct than political profile.

Leading is better

Despite the perils in leading the pack, though, it’s a lot better to be the front-runner than one of the other contenders. More often than not, front-runners prevail and win their party’s nomination: Walter Mondale in 1984, George H.W. Bush in 1988, Bob Dole in 1996, George W. Bush and Al Gore in 2000 are among many examples.

This pattern is particularly strong among Republicans, who, at least in presidential politics, are temperamentally more inclined to respect customary lines of authority and the prerogatives of party elders. Three of their last four elected presidents were respected party figures who had run for the White House before, and the fourth (George W. Bush) was the son of a president, the grandson of a senator, the brother of a governor and himself the governor of the largest state headed by a Republican. Indeed, every Republican nominee for the past four decades, with the exception of Bush and Gerald R. Ford, an appointed but incumbent president, had run for president before.

This factor, along with Romney’s bulging campaign treasury and the deference the other candidates displayed last week, underlines Romney’s position — a position so commanding that he actually outpolled Obama in the latest Washington Post-ABC News Poll. Indeed, Romney has more than four times as much support in New Hampshire as his nearest rival, former New York Mayor Rudolph Giuliani, who isn’t even in the race (yet).

New Hampshire is key

New Hampshire is vital for Romney, who owns a vacation home in the state and whose primary residence is in a neighboring state. His defeat there four years ago at the hands of Sen. John McCain represented a substantial repudiation, as Massachusetts politicians from John F. Kennedy’s time on have prevailed in New Hampshire — including Ambassador Henry Cabot Lodge (1964), who didn’t even campaign and managed to beat Sen. Barry Goldwater and Gov. Nelson A. Rockefeller in a remarkable write-in effort, and former Sen. Paul E. Tsongas (1992). The only exception, besides Romney, was Edward M. Kennedy, who was defeated by an incumbent president, Jimmy Carter, in 1980.

This time Romney simply cannot afford to lose the state, especially since he has all but abandoned efforts in Iowa, whose caucuses will precede New Hampshire’s primary by eight days. He can explain away a loss in Iowa: He won’t have competed, and the contest there is shaping up as a social-conservatism derby that is not to his tastes nor to his strengths. But losing New Hampshire, where he is well-known and where conditions favor him, would be fatal to his presidential hopes — especially since South Carolina, another social-conservative state, is likely to hold the next primary.

Romney was stung in Iowa in 2008 by former Gov. Mike Huckabee of Arkansas, which was bad enough, but losing in his home region to McCain five days later was too much for his campaign to absorb.

The big talk in the last days of spring is the way former Gov. Tim Pawlenty of Minnesota linked Obama’s health care plan, which is anathema to Republicans, to Romney’s plan in Massachusetts by speaking of “Obamneycare.” Pawlenty used the word again at the debate last Monday at St. Anselm College, but its long-term effect may be to elevate Romney into a political if not moral equivalent of the president — not the sort of tactic that always benefits a challenger.

Health care is top issue

The health care issue underlines Romney’s biggest task, which is to counter the notion that he is willing to change his views with the political breezes. His opponents believe he supported legalized abortion and a health care plan in Massachusetts because the political atmosphere in the state demanded it, and that he abandoned those views once he ascertained that the national political atmosphere required him to do so. Then again, he is being pilloried for his failure to change his mind on global warming.

The result is a political conundrum for Romney as great as any front-runner has had since Mondale was tarred as being the instrument of traditional Democratic special interests, especially organized labor. If Romney adjusts his position on global warming, he will be declared an opportunist. If he doesn’t, he will be at odds with a major tenet of the modern Republican creed.

Right now Romney is determined to assure that Republicans don’t change their minds. He has the biggest advantage in the Gallup poll that any Republican contender has possessed in the race. That margin will shrink. The story of the 2012 Republican presidential nomination fight will be how much it shrinks, at what rate it shrinks and whether it disappears. Such is the advantage and the danger of front-runner status.