EXETER, N.H. ? No Chris Christie. No Jeb Bush, no Mitch Daniels, no George Pataki. No Paul Ryan either. Nor Sarah Palin. There’ll be no reset for the Republicans. The GOP presidential field is set.

So when the Republican candidates square off at Dartmouth College Tuesday night there will be no echoes from the dog that didn’t bark. There is no such dog, he has no bark and, because that seems beyond doubt, Tuesday’s debate likely marks the beginning of the last phase of the campaign before balloting commences.

This new phase, which will position the candidates before the first caucuses in Iowa and the first primary in New Hampshire, provides the two leading contenders with the challenge of sharpening their positions, solidifying their bases and maneuvering for advantage in the fight for a presidential nomination that, with President Barack Obama’s poor ratings, is an even shinier prize than it might otherwise be against an incumbent. It also provides the second-tier candidates with their last, best chance to position themselves to take advantage of the front-runners’ stumbles or to create advantages for themselves.

Critical campaign period

For all of the reasons that the fantasy presidential campaign of Christie faced formidable obstacles — the divisions among Republicans, the strong position of former Gov. Mitt Romney of Massachusetts, the lingering appeal of Gov. Rick Perry of Texas, even the unlikely rise of businessman Herman Cain — the Tuesday debate and the six weeks between now and the beginning of the holidays are an unusually critical period. The chances of an insurgency prevailing after the turn of the year aren’t great. The seeds planted in October and November are the only crops ever harvested in the winter months in Iowa and New Hampshire, whose presidential contests now increasingly are likely to be held in early to mid-January.

Christie is gone but not forgotten. He left behind one legacy, one of the greatest withdrawal lines of all time, when he proclaimed that he dreaded the notion of getting up before dawn on a frosty morning to shake hands at a meatpacking plant in Iowa. That won’t dog him permanently. Sen. Walter F. Mondale said in 1974 that he lacked “fire in the belly” and couldn’t contemplate an endless series of overnights in Holiday Inns; a decade later, having served as vice president, he won the Democratic presidential nomination.

But the Christie boomlet, one of the shortest on record, highlighted the challenge facing those who remain in the race.

Christie was admired for his ferocious honesty, an attribute of Perry but not of Romney. He married that pugnaciousness with a certain probity, an attribute of Romney but not of Perry. He had appeal for establishment Republicans — few GOP governors of New Jersey, a suburb of Wall Street, fail to ease the worries of the business world, though Gov. James McGreevey, who served from 2002 to 2004, made an honest effort to disprove the rule. And he was potential balm for Republicans who in this season favor a cup of tea party caffeine. The only one who comes close in the bluntness-braininess category is Cain, who is unproven in politics but unsullied by that deficiency.

Broken GOP hearts

The Republican race thus far has been a marathon of heartbreak hills. Some of the broken hearts were produced by harriers who hurried to the sidelines. Some came from runners who lacked stamina for the long haul. Some came from the deficiencies of the runners who remain — deficiencies that are stubborn metaphors for the contenders themselves.

Romney remains a candidate who possesses little passion and inspires even less. He is methodical but almost mechanical, a Republican thoroughbred weakened by one of his virtues, his thoroughness. Hardly anybody but the people who have worked for him can work up any enthusiasm for him. He may win because he can. In a political year when the opponent is a weakened, struggling president that may be enough.

Perry’s problem is the opposite. He has enormous passion and inspires even more. He is no thoroughbred, which is at once his greatest asset and weakness. (Americans want their kids to go to Harvard Business School, but not their presidents; the only other Baker Library presidency, that of George W. Bush, did not end well.) But Perry still has a hunting lodge to explain and a Clayton Williams narrative to live down. Williams, so much an Aggie that he flew around in a plane painted in Texas A&M maroon-and-white, burst on the Lone Star political scene in 1990, the big-talking challenger in the gubernatorial race to then-state Treasurer Ann Richards. He was defeated narrowly. This analogy has limits, of course, as Romney is no Ann Richards. But Perry still must display a depth of thought to accompany his depth of feeling.

All that, plus early indications that voters peeling away from Perry may be pulled into Cain’s growing orbit, makes this week’s debate among the eight — a “mob scene” in the words of Dartmouth political scientist Linda L. Fowler — a vital confrontation. The emphasis is to be on the economy, and the candidates will be asked questions about how to improve conditions for middle-class voters, an opportunity for all the contenders to move the party away from the notion, buttressed by Obama and Democrats on Capitol Hill, that the GOP is little more than a gussied-up redoubt for the rich.