A victory over Rick Santorum in Wisconsin on Tuesday would effectively close the first phase of the primary season, senior Republicans say. It would leave Mr. Romney with not only a commanding lead in the race for delegates but also a claim to have fended off energetic challenges across a range of battleground states with a disciplined and well-financed campaign effort.
Mr. Romney and his aides continue to work behind the scenes to win support from respected voices in the party and prepare in earnest to take on Mr. Obama. The campaign will soon start raising money for the general election, donors said, as well as dramatically expand its Boston headquarters and build state operations across the country.
The Romney campaign is also taking steps to infuse the organization with well-seasoned advisers, Republicans said, and intensifying its research of prospective running mates.
"There's been plenty of opportunity for debates and discussion," said Gov. Terry E. Branstad of Iowa, who is among the Republican leaders eager to unite the party and concentrate on trying to win the White House. "We need to acknowledge and recognize that nobody is perfect, but there is going to be a very clear choice with the present administration."
Mr. Branstad, who has remained neutral in the race, is one of several governors and party officials who say a victory in Wisconsin — where Mr. Romney leads in the polls — would be the time for Republicans to rally behind Mr. Romney and call for the race to reach a swift conclusion. Party elders are discussing ways to help characterize Mr. Romney as the presumptive nominee — perhaps by the end of April — well before he reaches the 1,144 delegates needed to win the nomination.
That effort was helped along by the billionaire casino executive Sheldon Adelson, who, after sending $15 million of his family's fortune to the "super PAC" supporting Newt Gingrich, was quoted by The Jewish Journal of Greater Los Angeles last week as saying Mr. Gingrich seemed to have reached "the end of his line."
Mr. Adelson did so after telling a number of Romney financial supporters who were at his Las Vegas mansion for the opening dinner of the annual meeting of the Republican Jewish Coalition that he was ready to do whatever he could to help Mr. Romney become president and to deliver the Senate to the Republicans. (The dinner was first reported by the Center for Public Integrity, a nonprofit investigative news organization.)
An attendee of the private dinner said in an interview that the general sense there was "that the page had clearly been turned" and that it was time to focus on the general election.
Yet while Mr. Romney and his aides are said to be growing increasingly comfortable — allowing their fund-raising team to begin taking the legal steps necessary to raise money for a general election fund — they remain mindful that his rivals retain some chance, however small, of preventing him from gathering enough delegates for the nomination.
With Mr. Gingrich stepping aside from the daily combat of the race, Mr. Santorum remains the last real roadblock to Mr. Romney's becoming the nominee. Mr. Santorum is waging a tenacious battle in Wisconsin, bluntly reminding Republicans to send a message that the party needs a stronger conservative candidate than Mr. Romney.
But even as the three candidates campaigned on Saturday outside Milwaukee, where more than 1,000 party activists were expected for a gathering of social conservatives, aides to Mr. Santorum said they were just as focused on the action behind closed doors in North Dakota and Washington, where state and county gatherings were being held to select delegates for the Republican National Convention in Tampa, Fla., in August.
Michael Biundo, Mr. Santorum's campaign manager, said their team had always been anticipating a much better climate in May, when several states similar to those Mr. Santorum has won this year will vote. While the campaign remains confident about must-win Pennsylvania, which Mr. Santorum represented in Congress and which will vote April 24, it is also quietly picking up support from as-yet-uncommitted delegates who do not believe Mr. Romney is sufficiently conservative, Mr. Biundo said.
"The Romney campaign would love to close this out and take the ball home with them, because they know May is going to be a great month for us," he said in an interview. "What happens on the ground and what happens in the national media are sometimes two different things."
But since Mr. Romney's victory in Illinois on March 20, a parade of prominent figures has endorsed him and called for an end to the primary process.
The latest, Representative Paul D. Ryan of Wisconsin, chairman of the House Budget Committee, had not planned to pick sides in the race before the primary. But on Friday he endorsed Mr. Romney and warned Republicans that they did not have the luxury of waiting until the party's convention to unify. He said he was not troubled by a series of missteps that critics have used to suggest Mr. Romney is out of touch with voters. In the latest example, Mr. Romney joked last week about the time his father, as the head of American Motors, "closed the factory" in Michigan and moved jobs to Wisconsin.
"Whether a candidate has a gaffe on Tuesday or Thursday is not going to decide the campaign," Mr. Ryan said in an interview. "Voters are looking for someone who will show them a contrasting vision to the one that Obama has given us."
Even though 29 states have held their primaries and caucuses, only this week is Mr. Romney expected to pass the halfway mark to 1,144 delegates. According to an estimate by The Associated Press, Mr. Romney has 568.
A series of wins in the April primaries in states including Wisconsin and New York could add to his tally greatly.
"I don't think he's presumptive just yet, but I do think we're near a tipping point," said Ed Gillespie, a former Republican Party chairman and White House counselor to President George W. Bush who supports Mr. Romney. "April is pretty happy hunting ground for him, and he's got some potentially significant delegate accumulation ahead."
All the same, the protracted Republican primary season has paid dividends for Mr. Romney. At fund-raisers last week in California and Texas, donors said, he raised more money than he had initially expected. A swing through Dallas, Houston and San Antonio netted more than $2 million.
"It's made people very willing to give — just to get on with the next stage of the race," said Ray Washburne, a Dallas businessman and top fund-raiser who had dinner last week with Mr. Romney. "A lot of people who had been holding back are like, 'We're ready to go.' "
