Mitt Romney, mired in a frustrating slog toward the presidential nomination, has directed his top advisers to launch a multi-pronged effort to unite the Republican Party as the primaries draw to a close.

While Romney’s public activities are geared almost entirely toward winning the nomination, his campaign is on a private mission aimed beyond spring. Senior aides and surrogates — hoping to seize a moment when even some unfriendly Republicans are beginning to see Romney as inevitable — have spent the past several weeks making calls and visits to conservative leaders and activists who have resisted Romney’s candidacy.

Campaign manager Matt Rhoades has been wooing tea party organizers in general-election swing states and, in some cases, offering private meetings with Romney. Advisers with roots in the conservative movement, such as Bay Buchanan and Mark DeMoss, have been reaching out to evangelical leaders who are backing other candidates. Elected officials, led by Sen. Roy Blunt (Mo.), have been courting lawmakers and Beltway power-players who are not on board.

In Boston, longtime confidants Beth Myers and Ron Kaufman are developing a plan for the GOP convention in Tampa, to prevent the party from being divided during the nominee’s turn in the spotlight. Still other Romney emissaries are in touch with mega-donors — including billionaire Sheldon Adelson, who has been supporting former House speaker Newt Gingrich (Ga.), and Foster Friess, who backs former senator Rick Santorum (Pa.) — to persuade them to jump ship.

The coordinated approach, detailed this week in interviews with Romney advisers and supporters, is designed to assure all factions within the party that, as one adviser said, “when the dust settles, we want everybody on board.”

But the extent of the effort reflects just how daunting a challenge Romney faces: Although there has been a slow acknowledgment — even among tea party activists hostile to the former Massachusetts governor’s candidacy — that he is most likely to become the nominee, many remain unmoved to rally around him.

“President Obama’s a worthy campaign adversary, and Mitt Romney is going to need a very active grass-roots base that’s not only showing up to vote, but convincing their neighbors to do the same — and that deal is not sealed,” said Matt Kibbe, president of Freedom Works, a national group aligned with the tea party.

Among those being courted is Richard Land, a longtime leader of the Southern Baptist Convention. As a practice, Land said, he does not endorse political candidates, but he is considered a powerful barometer of the evangelical community.

Land said that after a private dinner with Romney last year at Acadiana, a Washington restaurant, Romney’s advisers have been in regular touch. Land said he recently told them that Romney could win over recalcitrant conservatives by picking Sen. Marco Rubio (Fla.) as his vice presidential running mate and previewing a few Cabinet selections: Santorum as attorney general, Gingrich as ambassador to the United Nations and John Bolton as secretary of state.

“They’re taking notes,” Land said. “I don’t know what they do with the notes, but they give a good impression of listening.”

Romney’s team also is focusing on the mechanics of waging a coast-to-coast fall campaign against Obama. Political aides have laid plans to deploy staff to general-election battlegrounds and opened lines of communication with the state parties and Republican National Committee in preparation for a coordinated effort.

In their conversations, Romney’s representatives are seeking a balance between prudently preparing for a general election and appearing too presumptuous.

“It’s never a great idea to count your chickens before they’re hatched,” said Virginia Gov. Robert F. McDonnell, who has been campaigning with Romney. “There are still other primaries, and the governor needs to stay focused on making sure he campaigns hard and wins the nomination.”

Ralph Reed, a prominent social conservative, said Romney’s team is in touch with constituency leaders, but it has conveyed “no real heavy pressure.”

“I don’t think they’re out there actively trying to run anybody else out of the race,” Reed said. “I think that would be a fool’s errand, and they recognize that.”

Still, Romney’s representatives deliver the same message in private that the campaign projects in public: The math doesn’t lie, Romney’s delegate lead is nearly insurmountable, and when he secures the nomination, he needs the party united to defeat Obama.

“At some point, probably in the near future, it’s going to become fairly obvious that Governor Romney’s the likely nominee,” said Nebraska Gov. Dave Heineman, who has been singing Romney’s praises to fellow governors. “I tell them, ‘Support the candidate of your choice in the primary.’ I’ve always been a big believer that competitive primaries strengthen our party. But when the primary’s over, let’s remember what we need to do, and that’s unite behind our nominee.”

Although Santorum’s strength promises a long and potentially ugly march for delegates, Romney’s team projects faith that the party will coalesce.

“One of the main unifying dynamics for the Republican Party and the conservative movement more broadly is going to be their mutual interests and passion in defeating Barack Obama. . . . That will be very powerful and magnetic,” said former Minnesota governor Tim Pawlenty, who became a top Romney surrogate after dropping out of the race last year.

Idaho’s C.L. “Butch” Otter, the first governor to endorse Romney, said the key will be channeling anti-Obama energy behind Romney.

“I just see a whole lot more energy and enthusiasm than I can remember in a long time in our Republican base,” he said. “We’ve got to sustain this energy and enthusiasm to convince people that Romney is the solution that we need to stop some of these crazy federal policies.”

Santorum’s advisers and supporters, buoyed by recent wins, said they have been making similar calls, although it appears to be a far less organized effort than Romney’s.

“We’re looking ahead at how we will do things as the nominee,” Santorum adviser John Brabender said. But he added that if Romney wins the nomination, Santorum and his team would support him. “Any Democrat or anyone in the press who thinks this will be a fractured party is dead wrong. The one thing that Barack Obama is is the great unifier.”

The Romney team isn’t counting on the fissures in the Republican base to heal easily on their own. “There’s a dozen different friends of Mitt out there making sure everybody understands that we’re not holding grudges and, when the dust settles, we want everybody on board,” said one adviser who, like some others interviewed, spoke on the condition of anonymity to discuss the conversations.

Romney’s representatives are making clear that new supporters will have a seat at the table, and they are asking what Romney and his campaign can do to erase doubts. They also are urging new supporters to endorse Romney for substantive reasons, not just because he seems like the eventual winner, hoping that will rally more grass-roots support.

“ ‘I’m happy to be with Romney because he’s the nominee’ is like saying, ‘I’m happy to be at the dentist because my tooth hurts,’ ” said another Romney adviser.

A top priority is soothing tensions with Adelson and Friess, who have bankrolled super PACs supporting Gingrich and Santorum and have the capacity to do the same on Romney’s behalf.

After Romney met last month with Adelson, one adviser said, some of his more pro-Israel surrogates have made regular contact with the Las Vegas casino mogul and have won his assurances that he would back Romney and give to the pro-Romney super PAC in the fall.

Among the party leaders not on board with Romney is Iowa Gov. Terry Branstad. He seems like a natural Romney ally — the two are close and share similar political bases, and several Romney advisers, including Kaufman, are longtime friends of Branstad’s.

But Branstad, noting that he has publicly complimented Santorum, said he is unaligned for a strategic reason: to be a credible broker.

“I want to play a role in uniting the party,” he said, “when it becomes obvious who our eventual nominee is going to be.”