Now, his team is using math to make the case to his rivals that they should drop out.
The team's argument: It's impossible to conceive of any scenario in which Rick Santorum or Newt Gingrich gets more delegates than Mr. Romney and wins the nomination in his place. And for the good of the party – and to start coalescing around someone who could beat President Obama in November – they should drop out now.
"Super Tuesday dramatically reduced the likelihood that any of Governor Romney's opponents can obtain the Republican nomination," Rich Beeson, Romney's political director, told reporters Wednesday. "As Governor Romney's opponents attempt to ignore the basic principles of math, the only person's odds of winning they are increasing are President Obama's."
Romney aides said they would be pressing hard behind the scenes for both Messrs. Gingrich and Santorum to drop out. (Ron Paul, whose goals are somewhat different, will almost certainly remain until the end.)
The Romney campaign's calculations: The former governor is far ahead of the other candidates in terms of delegate count now. Although counts aren't final because some delegates are unpledged, he currently has about 415 delegates – more than 35 percent of the 1,144 needed to secure the nomination. Santorum and Gingrich have 176 and 105, respectively, and Representative Paul has 47.
Moreover, there are few big states left like Arizona and Florida that are winner-take-all. Thus, even in states that Romney loses, he's likely to win a large number of delegates (given the closeness of the races so far). Plus, the few states that are winner-take-all – like Delaware, Utah, and New Jersey – are ones that seem more favorable to Romney.
Is Romney's team right? And, if it is right, how likely would it be able to persuade Santorum and Gingrich to drop out?
After a look at the numbers, it's certainly hard to imagine a nominee other than Romney – but it's not totally inconceivable. At the rate he's going, it will take him a long time – at least through May – to rack up enough delegates to clinch the nomination. A lot can happen in that time.
Even if it's impossible for Santorum or Gingrich to overtake Romney and win the nomination, they could prevent him from getting the majority he needs, which would force a brokered convention. (This has always seemed unlikely, but some political pundits and GOP insiders have been salivating about it – and even hoping for it – for several months.)
A brokered convention could possibly free up party leaders to tap someone other than Romney. A long-shot scenario, but a possibility.
"When one tries to do delegate projections, assuming that current voting patterns continue and taking into account various Santorum ballot access problems, one finds that Romney will probably continue to hover at a bit over 50 percent of the delegates chosen. He'll clearly be in the lead. But it's hard to see him amassing so insurmountable a lead by mid-May so as to be able to discourage Santorum from hoping to be able to catch him at the end with a huge victory in Texas on May 29, and then big upsets in California and New Jersey on June 5. Unlikely? Sure. Impossible? I don't think so."
All of which makes the Romney team's argument to Santorum and Gingrich a tougher sell.
"What Romney is trying to do is call the game before it's even half time because he has a lead," Santorum adviser John Brabender told CNN, dismissing the notion that the nomination is already decided.
Gingrich, far more than Santorum, seems to have few reasons to stay in the race. His only wins are Georgia, his home state, and neighboring South Carolina. He failed Tuesday to even notch a second-place finish in Tennessee or Oklahoma, both states where he seemed to be well positioned to beat Romney.
On Wednesday, Santorum's supporters were mounting pressure of their own for Gingrich to drop out.
"With Gingrich exiting the race it would be a true head-to-head race and conservatives would be able to make a choice between a consistent conservative in Rick Santorum or Mitt Romney," said Stuart Roy, an adviser to a pro-Santorum "super PAC," in a statement. "For instance, with Gingrich out of the race, Santorum would have won both Ohio and Michigan. Newt has become a hindrance to a conservative alternative."
But don't expect Gingrich to leave anytime soon. He has made clear his intention to stay in the race, telling supporters Tuesday night, "I'm the tortoise; I just take one step at a time."