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Former Utah Gov. Jon Huntsman Jr., Rep. Michele Bachmann of Minnesota and former Sen. Rick Santorum of Pennsylvania also remain in the fight, though Mr. Huntsman remains stuck in the low single digits in polls, and Mr. Santorum and Mrs. Bachmann have not been able to build on their isolated victories or minor poll surges.

The crowded field alone is enough to suggest a long haul ahead for the candidates, but some say the crowded field helps Mr. Romney by preventing the forces unhappy with his candidacy from coming together behind a single rival.

“The longer this continues to be a five-plus person race, the more likely it becomes that Romney is the nominee,” said former Virginia GOP Chairman Jeff Frederick. “Should one of the others be able to keep up with Romney in the marathon and benefit from some of the others dropping out, that candidate could be a realistic alternative to Romney, but that may be the only way Romney doesn’t get the nomination.”

Even if candidates drop out, their supporters are not expected to migrate to the same rival candidate, advisers to these campaigns say privately.

Hawaii’s Mr. Lee noted that a long, drawn-out race has become more plausible because of actions by a number of state parties — including in Florida, Arizona and Michigan — to ignore Republican National Committee guidelines and move up their primary dates to early in 2012 in hopes of playing a kingmaker role earlier in the nomination fight.

But jumping the gun will cost them half their regular delegate numbers — and make it harder for a hot candidate to run up the delegate count and drive others from the field, as Mr. McCain did in 2008.

RNC rules also call for subsequent primary and caucus states after the first wave of states to divide their delegates among the candidates instead of adopting the winner-take-all format that Republicans historically have favored.

As a result, it is mathematically impossible for any candidate to accumulate the requisite 1,142 delegates for nomination before March 24, even if that candidate won every vote in every contest from Iowa onward. Unless one candidate emerges to score an early knockout, the earliest practical date anyone can reach the magic number is well into April and possibly later.