As Mitt Romney's aura of inevitability grows, and conservatives yearn for Rick Santorum to get one last chance to challenge Romney one on one, Newt Gingrich is facing mounting calls to drop out of the race for the Republican presidential nomination. The former House speaker insists that he intends to stick around and pick up enough delegates to keep Romney from collecting the 1,144 he needs to seize the nomination outright. But Romney backer and 1996 GOP nominee Bob Dole says Gingrich's campaign is "probably finished, or almost finished," and many party insiders agree. What will it take for Newt to quit? Here, four theories:

1. He'll quit when he runs out of cash
Gingrich's campaign is "just plain broke," says Dave Levinthal at Politico. Team Gingrich spent more in February — $2.9 million — than it raised, and at last count, had more debt than cash on hand. Compounding that "dreary financial reality"? Newt now has dozens of creditors who are surely hounding him. Gingrich still has fans, and a message he wants to get out, says John Hinderaker at Power Line. But "he may well drop out." And if he does, "it will be because he can't pay his bills."

2. He'll quit when Mitt offers him a job
The campaign has restored Gingrich's long-lost "big-cheese status," says Michelle Cottle at The Daily Beast, and he doesn't want his "subsidized road trip" to ever end. That puts him in a "solid bargaining position" to demand something from the ultimate nominee. He might call it quits if Romney offers him an enticing post in a GOP administration — but it can't be something as big as a cabinet post, as that would scare independents into President Obama's camp. Still, it must be something befitting his "larger than life character." Head of moon colonies at NASA, perhaps?

3. He'll quit when he loses at the convention
Even after his last-place finish in Tuesday's Illinois primary, Gingrich says he plans to stick around until the Republican convention in Tampa. "As Gingrich sees it," says Steve Benen at MSNBC, "if no candidate has the 1,144 delegates needed to claim the nomination" when primary voting ends on June 26, things won't be settled until the August convention. Of course, that's hardly a winning plan — GOP rules say only candidates who have won five states can even earn a spot on the first ballot at an open convention. Gingrich has only won two so far, and his chances of picking up another three grow dimmer as Romney appears more and more inevitable.

4. He'll quit any day now
"Gingrich will soon drop out," says Steffen Schmidt at the Des Moines Register. "He is no longer the 'hardy warrior' fighting the 'good cause' for conservatives. He's become a 'pain in the butt.'" Gingrich's voters might start abandoning him in Saturday's Louisiana primary, says Stephanie Condon at CBS News. Many seem to be concluding that Santorum is the best shot conservatives have to beat the moderate Romney. Since Gingrich was staking his bid on the South, another loss or two there might get him to face reality.