The playbook for Republican presidential contenders goes at least as far back as Richard Nixon: Run hard to the right in the primaries; steer back to the center for the general election.

Former Massachusetts governor Mitt Romney has sold himself all along as the most electable Republican in the field. But as he moves closer to becoming the GOP standard-bearer, the candidate whose authenticity has been questioned from all sides faces a tricky challenge making a pivot for the next phase of the race.

Move toward the center, he infuriates the base; refuse to, he will alienate independent voters. And however he maneuvers, will voters be left with a clear picture of why he is running? Nothing is more central to the GOP self-identity than that this is the party that stands for big ideas.

As Romney has tried to win over his purist skeptics in the GOP’s activist base, he has shifted to the right on a number of issues. Most notable has been his hard-line stance on illegal immigration, where he outflanked Texas Gov. Rick Perry and former House speaker Newt Gingrich.

Among the other issues that could put him at odds with independents: his increasingly explicit support for the House Republican plan to restructure Medicare; his declaration that the housing market should be allowed to “hit the bottom,” rather than backing government intervention to slow the rate of foreclosures; his criticism of President Obama’s possible move to end the combat mission in Afghanistan more than a year ahead of schedule.

“Romney’s twin challenges are to unify the Republican base, where significant elements remain unconvinced of the strength of his conservative philosophy, while at the same time not genuflecting so much that he can’t appeal to the independent vote that will ultimately decide the election,” said Ken Duberstein, a former chief of staff in Ronald Reagan’s White House.

In the view of some in his party, Romney has an additional — and more serious — problem heading into the general election: He has thus far failed to brand his candidacy with an expansive vision.

“The fundamental question is whether Romney’s leadership can shape the Republican Party or will the far, far right define Romney?” Duberstein said.

A 59-point economic plan, some senior Republicans point out, is not the same thing as a big idea. Nor is Romney’s constant recitation of the lyrics of “America the Beautiful.”

Argument could lose edge

Going into the fall, Romney will no longer have the advantage of superior resources and organization. And if the economy continues to improve, his most potent argument — that Obama is simply not up to the job of fixing it — will lose its edge.

“What worked against an underfunded Mr. Gingrich won’t work against the well-funded Barack Obama,” Karl Rove, former president George W. Bush’s chief political strategist, warned in a column in the Wall Street Journal after Romney’s Florida win. “He should become bolder in his prescriptions, presenting a confident agenda for economic growth and renewed prosperity through reforms of taxes, regulatory and energy policies.”

The Republican base may be coming around, as evidenced by Romney’s solid victory in Florida and his expected win in Nevada.

But the longer the battle for the nomination goes on, the harder it may be for Romney to find his groove for the general election. Between his more conservative pronouncements and the corrosive tone of the primary race, Romney’s standing has fallen with independent voters, the group that will, in likelihood, determine the outcome in November.

In a Washington Post-ABC News poll, released after the South Carolina primary, just over half of independents said they have an unfavorable view of Romney, a number that rose by more than 20 points since late November.

Meanwhile, the share of independents who have a positive impression of Romney dropped to 23 percent, from 45 percent, over that same eight-week period.

‘In a vise between the two’

Already, there are signs that Romney is trying to strike a more delicate balance between the exigencies of the primary race and the challenges that await him if he succeeds in getting the nomination.

In Florida, for instance, “Romney signaled a turn toward the general election when he endorsed the idea of military service as a pathway to immigration,” said Mark McKinnon, who was a media adviser to Bush in both of his successful presidential campaigns. “But he hasn’t coalesced conservatives, and independents have turned against him. So he’s in a vise between the two, which will make recalibration very painful.”

The difficulty is compounded by the fact that Romney is not the most agile of candidates, as evidenced by a string of gaffes that, most recently, included what he conceded was a misstatement: “I’m not concerned about the very poor.”

Those kind of comments — and the attention that his opponents have drawn to his wealth and his now-closed Swiss bank account — may have aggravated the concerns of the 45 percent of independents who said, in a January poll by The Post and the Pew Research Center, that Romney does not understand the problems of average Americans. Only 34 percent said they believe he does.

And moments such as Donald Trump’s endorsement Thursday may come back to haunt a candidate who promises steady, sober leadership.

With his questioning of the authenticity of Obama’s birth certificate, Trump has built an enthusiastic following among the “birther movement,” which contends falsely that the 44th president is constitutionally ineligible for the office.

But polls have shown that Trump’s endorsement is, on balance, a turn-off for voters — and it would also put Romney on the hook for whatever outrageous statements Trump makes between now and November.

Obama’s reelection campaign does not plan to make the adjustment to the general election campaign any easier for Romney.

At its headquarters in Chicago, Obama’s team is compiling a dossier of Romney statements made in the heat of the primary battle.

“I know he walked away with the hard drives in Massachusetts,” said Obama’s chief political strategist David Axelrod, referring to the fact that Romney’s gubernatorial staff took computer records with them when they left office. “But the video [from the primary campaign] is going to be hard to erase.”

About even in swing states

Romney aides declined to discuss any plans to retool their campaign for the general election, saying that talking about it amid a primary race would be presumptuous.

They note that, whatever the challenges ahead, Romney is already running just about even with Obama in the swing states — something that is true of no other GOP contender.

And some Republicans note that Romney has always seemed more comfortable when he is on a general election footing, training his fire on Obama and focusing on his own spectacularly successful record in business.

Indeed, his initial strategy for winning the nomination had been to all but ignore his GOP opponents and run as if he already had the nomination.

“Romney’s strength is as a general-election candidate,” said Rob Stutzman, a California-based Republican political consultant. “In some ways, he’ll be able to unleash the real Romney.”

“He’s a good candidate when he engages Obama,” Stutzman added. “He tried to get nominated that way, and it didn’t work.”

Stutzman and others argue that the sooner Romney can get to a two-man contest with the president, the better. But Romney himself professes not to be concerned by the fact that the primary race is likely to drag on at least until next month’s Super Tuesday contests.

“A competitive primary does not divide us,” Romney said in his Florida victory speech. “It prepares us.”

Polling director Jon Cohen and polling analyst Scott Clement contributed to this report.