On Wednesday, the famously genial Mitt Romney sent his fellow Republican candidates a steel-hard message.

Give up, the message went. Resistance is futile.

Romney’s aides laid out a detailed case, saying that after the former Massachusetts governor won six states on Super Tuesday, it is now nearly impossible for anybody else to capture the GOP presidential nomination.

Romney, they said, is too far ahead in the all-important race for delegates.

Because of that, the aides said, Rick Santorum and Newt Gingrich are only delaying the inevitable by continuing to campaign. And — worse yet — they could be aiding the enemy by muddying Romney’s image and emptying his campaign coffers before the general election in the fall.

“Super Tuesday dramatically reduced the likelihood that any of Governor Romney’s opponents can obtain the Republican nomination,” Romney political director Rich Beeson wrote in a memo to reporters. “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.”

That argument underscored Romney’s bittersweet position; appealing to math is nobody’s Plan A. Romney now seems almost guaranteed to win his party’s nomination, but he has struggled to capture the passion and imagination of its conservative base.

On Wednesday, however, there was little sign that his rivals were taking Romney’s hint.

“What won’t they resort to, [trying] to bully their way through this race?” Santorum asked Wednesday, rejecting Romney’s suggestion that he get out of the way. Santorum addressed more than 200 supporters at a graphics company in Kansas, where the next primary contest will be held. “If the governor thinks he’s now ordained by God to win, then let’s just have it out.”

Romney is unlikely to score a knockout blow in the next few contests. In fact, he seems likely to lose on Saturday in Kansas, and again on Tuesday in Mississippi and Alabama.

Wednesday’s message was an attempt to preempt those losses by laying out the logic of inevitability.

Despite what Santorum said, the argument wasn’t about divine will. It was about something almost as mysterious: the Republican Party’s method for doling out delegates to its August convention in Tampa. To win the nomination, a candidate must amass more than 1,100 delegates.

Romney has won about 415 — having added 211 more on Tuesday with victories in Alaska, Idaho, Massachusetts, Ohio, Vermont and Virginia.

His closest rival, Santorum, has less than half that total: 176 delegates. Gingrich is even further back, with 105.

To catch up, Romney aides said, those two would have to win delegates in big chunks in the weeks to come. Which is not likely.

That’s because there are only four jurisdictions left with winner-take-all contests, in which one candidate can collect every delegate. But all of them — Delaware, the District, New Jersey and Utah — appear to hold tremendous advantages for Romney.

In many of the other states, the winning candidate will have to share the delegates with his rivals. That would give Santorum and Gingrich little ability to make up ground.

“There’s not a lot of Floridas left out there, no more Arizonas, no more Virginias,” one of Romney’s senior aides said, listing delegate-rich states that the candidate has already won.“There’s just no more big chunks of delegates to go get.”

Wednesday’s briefing was held in a war room at Romney’s headquarters, with color-coded maps of Super Tuesday states still hanging on the wall. The session was marked by the campaign’s customary flair for overkill.

Aides, for instance, had analyzed the Republican races in Puerto Rico, American Samoa, Guam and the Virgin Islands. These U.S. territories will send a combined 59 delegates to the GOP convention. The conclusion was that Romney would secure them, too.

“They can’t win,” senior adviser Eric Fehrnstrom said of the other contenders, summing up the case. “I just don’t see how they overtake Mitt in the delegate count.”

Romney’s argument seemed to assume that his opponents would view their positions the same way he saw his own in 2008 — when, after grimly studying delegate counts and state projections, he bowed out to make way for Sen. John McCain (Ariz.), the eventual nominee.

But Santorum and Gingrich are different candidates — and vastly different minds.

Neither campaign made a detailed case Wednesday showing that Romney’s figures were wrong. Yet neither gave any indication of giving up. On Wednesday, Gingrich defended his decision to stay in the race — despite his delegate numbers — by saying that Romney isn’t up to the nominee’s job.

“I believe it is going to be impossible for a moderate to win the general election,” Gingrich told supporters in Montgomery, Ala. “We tried it in 1996 and it didn’t work. We tried it in 2008 and it didn’t work.”

The situation may look different, however, to the wealthy donors who have poured money into super PACs friendly to Santorum and Gingrich. Romney’s message seemed aimed at them as much as the candidates.

Also Wednesday, Romney’s supporters on Capitol Hill began making a similar pitch to colleagues who had pledged support for Romney’s rivals.

“It’s respectful to allow other candidates a shot,” said Sen. Rob Portman (R-Ohio), a Romney backer who said he had done this kind of lobbying. “I think it’s time for us to start coalescing around a candidate who can focus on the Democrats instead of Republicans attacking each other.”

Fahrenthold reported from Washington. Staff writers Felicia Sonmez in Kansas, Krissah Thompson in Alabama, and Aaron Blake and Ed O’Keefe in Washington contributed to this report.