Poor Mitt Romney. He won six of 10 states on Super Tuesday, including hotly contested Ohio. He lengthened his lead in the count of delegates who will actually choose the Republican presidential nominee. But he’s still a long way from claiming victory.

Why? Because there’s no compelling reason for Rick Santorum, Newt Gingrich or Ron Paul to drop out of the race. Each has a reason to keep fighting at least through April — and maybe all the way to the convention in August.

The elongated GOP primary race is partly a product of new party rules that aimed deliberately to produce a longer campaign, mostly by allowing losing candidates to win more of the delegates through proportional allocation. (“The dumbest idea anybody ever had,” in the assessment of New Jersey Gov. Chris Christie, a Romney backer.) And the marathon is partly because of “super PACs,” the new fundraising vehicles that allow billionaires to keep a favorite campaign on life support.

But it’s also a product of the individual circumstances of Romney’s rivals.

Santorum is the most straightforward case. He’s in second place in the delegate chase, and stands at least a theoretical chance of winning if Romney were somehow to collapse. But even more important, Santorum, 53, is the youngest of the four candidates; if he runs an impressive race and finishes second (or, as he put it on Tuesday, with “silver medals”), he would be a logical candidate for vice president this year — or president in 2016. “He’ll go all the way to the convention,” a Romney adviser predicted.

Paul is a little more complicated. The Texas congressman hasn’t won a single primary — not even in Alaska, a libertarian hotbed. But he’s running to promote a cause, not to win the nomination. He’s the oldest candidate at 76, but he has an heir — Sen. Rand Paul, R-Ky., 49 — who can pick up the banner next time. So Paul too has good reasons to stay in the race long enough to win as many delegates as he can — the better to get some libertarian themes into the GOP platform and claim a prime-time speaking role at the convention in Tampa, Fla.

Then there’s Gingrich, the most complicated of all. At 68, this probably is his one and only run for the presidency. And it’s increasingly clear he doesn’t have a shot. But of all the candidates, he’s the one who appears to be enjoying himself the most. He’s turned his campaign, with its eclectic lectures on energy policy and outer space, into a kind of performance art.

Gingrich says he’s confident he can win the next few contests and that, in any case, he intends to stay in the race until the California primary June 5. And there’s no reason he can’t, as long as billionaire Sheldon Adelson doesn’t cut off his super PAC’s cash flow.

Why? Posterity. Gingrich is unusually candid about comparing his career to those of Abraham Lincoln, Ronald Reagan and British Prime Minister Margaret Thatcher. He cares immensely about his legacy, and has still got some burnishing to do if he hopes to rank with his idols.

For all the candidates, staying in the race is a way to make sure they finish the contest with their reputations enhanced. And the most tangible prize they seek is a speaking role at the Republican convention.

You might wonder whether giving a speech on prime-time television is really all that crucial. But just ask Patrick J. Buchanan, who ran for the GOP nomination in 1992 and 1996 and lost both times:

“You bet it is,” Buchanan told me this week. “It’s an opportunity, if people have been describing you in a way you don’t think is right, to correct the picture. You can go down there and capture the country — unfiltered by the media. It is an unbelievable opportunity. It is an historic opportunity. Anybody who is offered it ought to grab it.”

In 1992, Buchanan made a deal with the nominee, then-President George H.W. Bush, to deliver a full-throated endorsement in exchange for a half-hour of prime time. Buchanan’s speech included the endorsement Bush wanted, but it was remembered mostly for Buchanan’s declaration of a “culture war,” a call to arms for social conservatives that many believe sent independent voters straight to the Democrats.

This time, Buchanan noted, whoever wins the nomination faces a ticklish problem: He’ll have three defeated challengers to appease — “and only one hour of prime time each night.”

“I think Romney’s going to win, but he really has to go to work as a diplomat to bring the alliance together,” he said. “Magnanimity will go a long way.”

But that’s a problem that can wait until summer. For now, Romney still needs to endure another month — or three — of battles. He needs to worry about the polls that show that his conservative rivals’ punches are convincing voters that he’s a wealthy opportunist who’s out of touch. He needs to close the deal, the sooner the better. Because the longer he’s forced to compete with Santorum, Ging-rich and Paul for votes from conservatives, the harder it might be for him to win votes from independents in the fall.

Poor Mitt.

Doyle McManus is a columnist for the Los Angeles Times.