If Mitt Romney cruises to victory in the New Hampshire primary on Tuesday as expected, the former Massachusetts governor will be on a glide path to the Republican presidential nomination — a path that could end as soon as the South Carolina primary on Jan. 21.

If Romney wins the Palmetto State, after winning in Iowa and New Hampshire, it’s hard to imagine anyone in the GOP field — with the exception of Rep. Ron Paul (Tex.) — hanging on much beyond the end of January.

That would help Republicans’ chances of beating President Obama in November, as Romney and the GOP would be able to focus their time and money on the Democratic incumbent rather than waste it on an intraparty skirmish. Or so the conventional wisdom goes.

But a look back at recent presidential nomination fights suggests that the conventional wisdom may be wrong — and that the best thing for Romney and the party’s chances in the fall may be a protracted fight for the nod.

“An extended primary would actually help Romney,” said Mark McKinnon, an unaligned GOP strategist. “It keeps the attention on Republicans, he looks stronger and a longer spring training means he’ll be in better shape for the regular season.”

Take the past two presidential primary fights.

In 2008, Sen. John McCain’s victory over Romney in the Florida primary at the end of January effectively ended the nomination fight. (Romney dropped out shortly thereafter and although former Arkansas governor Mike Huckabee remained in the race, McCain was widely regarded as the nominee.)

The Democratic primary fight between then-Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), on the other hand, stretched into June — an extended battle that had Democrats nervous and Republicans gleeful about what it meant for the general election.

And we know how that one turned out.

The 2004 presidential race teaches a similar lesson. After scoring wins in the Iowa caucuses (Jan. 19) and the New Hampshire primary (Jan. 27), Sen. John F. Kerry (Mass.) secured the Democratic nomination.

At the time, party strategists were thrilled with the outcome — the most electable candidate (in their view) had won a quick and resounding nomination fight and was ready to take on President George W. Bush. The reality was something very different as Bush and his political team quickly pounced, defining Kerry as a wishy-washy flip-flopper before the Democratic nominee was able to get out of the general-election gate.

Look further back in history and you find more evidence that longer is better than shorter when it comes to primary fights. Then-Vice President Al Gore swept to the Democratic presidential nomination in 2000 only to lose to Bush. (Bush’s 2000 campaign is something of an outlier to the theory, as he effectively ended it by beating McCain in South Carolina on Feb. 19 but went on to win the general election anyway.) Former senator Robert J. Dole (Kan.) had the nomination in hand by mid-March 1996 but was outspent by far in the spring and summer and went on to lose to President Bill Clinton in the fall.

Phil Musser, who helped Romney during his 2008 campaign and was intimately involved in the presidential effort of former Minnesota governor Tim Pawlenty this time around, said that no matter what happens, Romney won’t repeat Dole’s mistakes.

“Between the [Republican National Committee], the outside groups and the very extensive Romney finance network, we’ll have the money needed to compete, so we won’t be in a handcuffed situation like 1996,” he said.

Regardless of whether Romney could handle a short primary season, a longer one is likely to help more. Although more primaries mean more money spent and more time expended on a fight within one party, it also means scads of news media attention — the press would much rather cover an active race than one in which the ultimate vote won’t come for nine months or more — and the chance to run a series of real campaigns in states that will be competitive in the general election.

In practical terms, Romney and his team don’t care when they win the nomination — as long as they do win it. But a look at recent presidential history suggests that a quick victory may provide short-term gain in exchange for long-term political pain.

“It’s like sitting your starters in football,” Matt Bennett, who was a Clinton administration official, said of a short primary. “The rest may feel good and prevent injury, but it doesn’t steel you for the championship.” (Ask the 2009 Colts or John Kerry).