As President Obama’s campaign fired a shot against Mr. Romney with a new ad campaign attacking him as cozy with “Big Oil” — and Mr. Romney took formal steps to raise general election funds — Mr. Santorum kicked off an intense new drive to wrest the party mantle away from Mr. Romney before he secures the nomination for good. It was replete with the political equivalent of the nuclear option: A television commercial that morphed the face of President Obama into that of Mr. Romney. Throughout the day Mr. Santorum slammed “the establishment” members of his party for trying to end the contest before all the voters had spoken, and he mocked Mr. Romney as a moderate “Massachusetts governor” — “There’s a conservative you can stand by,” he deadpanned — whose positions on social issues have been “all over the map.” “With all the establishment and all the media singing this song, ‘Well, it’s over it’s, over it’s over,’ less than half the delegates have spoken,” he told a small but enthusiastic crowd gathered at a highway-side banquet space here. “We’re not even at halftime.” Mr. Santorum and his aides may now openly acknowledge that his odds for the capturing the nomination look high, and that a victory here would be a long shot (it is assumed by both sides he will lose in Maryland and the District of Columbia, which also vote Tuesday). But they are digging in for another month of hard campaigning at least, a reminder that for all of Mr. Romney’s progress, an ideological rift remains that Mr. Santorum will exploit as long as he sees a path to win, even if by forcing an open convention. And as long as he remains a threat, Mr. Romney’s campaign will be forced to keep attention and money focused on beating him back as much as on the larger task of defeating Mr. Obama, even as Mr. Obama’s campaign increasingly attacks him. Mr. Romney’s aides are plenty aware that while the likelihood that Mr. Santorum can block him from acquiring the 1,144 delegates may be small, it is not quite zero. So for all of the talk about the Republican primary season coming to a conclusion, Mr. Santorum, Mr. Romney and their supportive outside groups are campaigning hard here, spending at least $4 million on television ads — most of it from the Romney side — something that is likely to be repeated in the coming weeks. Keeping the pressure on Mr. Santorum as much as possible, Mr. Romney’s campaign has dispatched a traveling aide to forward an anti-Santorum message at his events here. The aide was asked to leave a Santorum appearance on Sunday, and did, but he attended the rally here Monday without complaint. That continuing dynamic is boiling down to a vital question for a party burning to replace Mr. Obama: When does Mr. Santorum’s slim hope become the party’s big problem? It is a question Mr. Romney’s campaign will press with increasing intensity through the coming weeks as it seeks to upset Mr. Santorum in his former home state of Pennsylvania on April 24 — and to pressure him into dropping out before he gets into what is presumably more favorable ground in May, when Southern states including Texas and North Carolina vote. “I just think we can get to the counterproductive stage of this primary,” Representative Paul D. Ryan of Wisconsin said after a campaign event in Howard, Wis., where he appeared with Mr. Romney. Worrying aloud that the prolonged primary season would set back the party’s need to match Mr. Obama’s fund-raising for the fall, he added, “It’s in our interest as a party to focus on the fall and the sooner we do that the better we’re going to be.” As if he heard Mr. Ryan’s admonitions, Mr. Santorum told reporters in Appleton, Wis., “People say going to the convention would be dangerous — no.” “I would argue even if it ends up in a convention, that’s a positive thing for the Republican Party,” he said. “That’s a positive thing for activating and energizing our folks heading into this fall election. I think in this primary, the longer it goes the better it is for the party.” A brokered convention, which he hopes to force, “would be energizing.” Mr. Romney’s campaign was clearly annoyed by the new advertisement from Mr. Santorum, which includes a debunked assertion about his position on carbon emissions and a disputed claim about his tax record. “Senator Santorum is at a point of desperation that he will say or do anything,” it said in a statement. “It is pretty clear that he is lashing out at everyone around him in order to prop up his sinking campaign.” The spot opens with a picture of Mr. Obama as a female narrator works through a series of policies he has presumably carried out: higher taxes, “big government mandating health care” and his support for “Wall Street bailouts” before revealing that it was not Obama she was talking about, but Mr. Romney. Hogan Gidley, a Santorum spokesman, said that the advertisement was running statewide in Wisconsin as the race there neared its end and that it was to be used in Pennsylvania, as well. Voters supportive of Mr. Romney said they were getting nervous that the damage was piling up. “He should stop,” said Jim Knutson, 82, a retired insurance salesman who attended Mr. Romney’s town hall-style meeting in Howard. “He’s not helping the cause.” But for now, at least, Mr. Santorum’s core voters do not seem to be losing their enthusiasm, with some saying it is Mr. Romney who should give up. “I say he should hang it up,” a Santorum supporter in Green Bay, Wis., Joe Draves, 35, said of Mr. Romney. “Santorum is the candidate of conservatives.”
