DES MOINES, Iowa — Mitt Romney faces a daunting to-do list as he transitions into the role of likely Republican presidential nominee.

Among the tasks: Raise as much money as possible for the general election campaign against President Barack Obama. Hire more people and send them to the most critical states in the fall race. Hone his message to appeal to voters across the political spectrum.

And do it all quickly while fending off challenges from GOP rivals who refuse to quit the primary race.

Obama, with the advantages of an incumbent, is well ahead of Romney on fundraising, organization and broad pitches to voters. So Romney can be expected to spend part of his time over the next three weeks trying to catch up. There’s a break in the primaries lasting until April 24, when several Northeastern states vote.

Romney also must start thinking about a running mate and strategy to amass the 270 electoral votes needed to win the White House on Nov. 6.

The former Massachusetts governor must prepare to put his imprint on the Republican National Committee and figure out how to achieve unity with a conservative base that has resisted his candidacy. In the general election, party loyalists will be counted on to raise money and get out the vote.

“I do think the Romney team is thinking about how they put in place their fall campaign,” said Terry Nelson, a former top aide to President George W. Bush. “But they clearly have some contests to get through, so they won’t be able to turn their eyes entirely to that.”

There’s little question that Romney will clinch the nomination in June, if not earlier. He has a wide lead in the race for the 1,144 delegates required to secure the GOP nomination. But chief rival Rick Santorum says he’ll press on at least through the end of the month. Pennsylvania, which he represented in the House and Senate, votes April 24, along with Connecticut, Delaware, New York and Rhode Island.

In hopes of convincing Republicans it’s time to rally behind Romney, leading Republicans such as Sen. Marco Rubio of Florida and Rep. Paul Ryan of Wisconsin have endorsed him recently; both are viewed as potential running mates. Former Rhode Island Gov. Don Carcieri has said he would back Romney and Iowa Gov. Terry Branstad may announce his support soon.

Romney is sounding more like a general election candidate each day. “It isn’t about one person or about even one party,” he said last week. “We’re Republicans and Democrats in this campaign, but we’re all connected with one destiny for America.”

After a break for the Easter holiday, Romney is expected to plunge back into fundraising in New York and South Florida. That’s none too soon for Republicans, given Obama’s fundraising advantage.

“Ultimately, the thing we have to focus on is getting the general election money raised,” said Brian Ballard, of Florida, one of Romney’s top fundraisers.

Obama, without a Democratic challenger, has been free to raise money strictly for the general election. He’s so far raised more than $300 million for his re-election campaign and the Democratic National Committee. Romney can’t raise cash for the Republicans until he clinches the nomination, but he’s brought in more than $75 million for his campaign.

Romney aides said solicitations for general election donations were starting to go out.

Obama showed his fundraising clout recently by spending about $1.4 million on TV ads in Colorado, Florida, Iowa, Ohio, Nevada and Virginia that criticize Romney. An outside group that supports Obama also is running ads attacking Romney in those states, as well as in Michigan and New Mexico.

Illustrating the disparity, Romney’s team responded to the Obama ad with a statement promoting an Internet attack video.

It’s not just money where Romney lags.

The president’s re-election team has opened offices and assembled teams of workers in Ohio, Florida and other critical states. The campaign has mapped out the combination of states it will compete in as it works to reach the 270 electoral votes needed for victory.

“No question Mitt Romney is the best-organized Republican I’ve seen in a long time, but that’s not the question,” said Florida Republican campaign strategist Susie Wiles, a senior Romney adviser in the state. “The question is not whether he is organized. It is whether he can identify his supporters and get them to vote better than the Obama people. I wouldn’t bet against him.”

Romney’s team is tight-lipped about how he can get to 270. It also won’t discuss when and where staff will go in the coming weeks or when it will run ads in the most contested states.

That’s probably because there isn’t a definitive plan — or maybe even a tentative one.

Republicans expect Romney will compete most vigorously in Colorado, Florida, Iowa, Ohio, Nevada, Virginia, Michigan and New Mexico, states considered among the most contested in the general election.

Obama won all eight in 2008 against Sen. John McCain, R-Ariz. In 2004, all but Michigan were carried by Bush during the Republican president’s re-election. Republicans say Romney sees Ohio, Michigan, New Mexico, Arizona and Colorado as fertile ground this year.

Romney aides say they’ve lined up staff to send out to states once Romney secures the nomination, and the campaign is preparing the first floor of its three-story Boston headquarters for the influx.

Some key players have returned to states, such as Romney’s Florida director Matthew Parker. And, aides point out, Romney has loyal activists and a network of supporters in key battleground states where he won primaries such as Florida and Ohio.

At the same time, the RNC is opening coordinated campaign offices in Florida and other battleground states, and has spent more than a year raising millions to support the eventual nominee. But Romney and the RNC are barred from coordinating until the nomination is in hand. Even so, the candidate and the party are entering a joint fundraising agreement to get ready for that day.

Once Romney seizes the nomination, he’s expected to have little trouble taking over the party, considering his campaign manager, communication director and political director all are RNC veterans.

But an alliance with the RNC doesn’t mean the party’s rank-and-file will automatically rally behind Romney.

Veteran GOP presidential campaign strategist Mary Matalin said Romney needs to anchor his schedule with “unifying events” that focus on the conservative establishment.

In a sign that Romney knows he has work to do on this front, he has scheduled an appearance before the National Rifle Association’s annual meeting on April 13.