With support for Mitt Romney eroding on the eve of the critical South Carolina primary, his campaign advisers are redoubling their efforts to consolidate elected officials, major donors and key activists from throughout the GOP establishment. And they are having some success.

Several prominent Republicans who until recently were neutral have endorsed Romney over the last few days. Among them are two GOP swing-state stars buzzed about as potential vice presidential candidates — Sen. Rob Portman (Ohio) and Virginia Gov. Robert McDonnell — who campaigned with the former Massachusetts governor in South Carolina this week.

“I think the best candidate to beat Barack Obama and to restore confidence in the United States of America is Mitt Romney and that’s why I’m here,” McDonnell told reporters after a rally with Romney in North Charleston on Friday. “I hope a lot of people come on board right now and let’s get this done and start focusing on the general election.”

Prominent bundlers — including John McCain’s 2008 finance chairman Fred V. Malek, who had been unaligned, and former Bush ambassador David Wilkins, a leading supporter — joined the Romney campaign in recent weeks, giving him a new network of fundraisers who have raised hundreds of thousands of dollars for past presidential nominees.

One of the big questions after Texas Gov. Rick Perry’s withdrawal has been whether his large stable of donors and endorsees would follow Perry’s lead and support former House speaker Newt Gingrich. Romney aides are not automatically surrendering that ground. On Friday, Romney announced the backing of three congressmen from the Lone Star state and his fundraisers are wooing hundreds of Perry’s top Texas donors.

“The logic of his candidacy is very powerful and suggests, yes, he’s gonna be the nominee,” said former senator Jim Talent of Missouri. “The people who know what it takes to run for an office like this, I think, have become increasingly convinced that he has the ability to do it — that he can win and that he can govern afterwards. It’s also increasingly obvious that the others that are left are not in a position to do that.”

Talent, a close Romney adviser, said one of the campaign’s fundraisers in California called him Friday morning to say he had recruited five Perry donors there. And in Texas, Talent said, “I don’t think they’re gonna move to Newt. . . . I’ve talked to people down there who say we’re going out to get them now.”

Still, many party establishment figures remain on the sidelines — among them, Indiana Gov. Mitch Daniels, who decided against a run last spring and is slated to deliver the Republican response to President Obama’s “State of the Union” address next week.

Aides to Gingrich sounded unimpressed with the Romney effort. Asked about Romney’s gathering of establishment support, Gingrich spokesman R.C. Hammond replied with a short e-mail: “Hasn’t done much so far.”

Gingrich received new endorsements boosts Friday, picking up support from movie star Chuck Norris and Michael Reagan, a conservative talk show host and son of former President Reagan.

Although the establishment imprimatur can help finance a campaign, it can backfire with voters — an outcome former Pennsylvania senator Rick Santorum is trying to stoke with a provocative ad in South Carolina. The ad, “Rebellion,” is modeled after George Orwell’s totalitarian novel “1984” and shows robot-like humans walking in straight lines as images of Romney hang over their heads.

“The establishment is once again telling us to fall in line . . . and vote for their backroom, hand-picked moderate candidate,” the ad’s narrator says.

Here in South Carolina, several key new supporters have signed on with Romney in recent days. They include investor Barry Wynn, a Perry supporter; former attorney general candidate Leighton Lord; and former adjutant general Stan Spears, a supporter of McCain’s who was encouraged to join the Romney campaign by the Arizona senator.

“We’ve got all these people wanting to get on the Romney train,” South Carolina Gov. Nikki Haley said as she introduced Romney on Friday at a rally in North Charleston.

Much of Romney’s support comes from the sense that his nomination is inevitable, but his aura of inevitability could be dashed with a loss to Gingrich in Saturday’s primary here. So Romney’s team has doubled down in recent days to lock down, and in some cases announce publicly, as many supporters as it could.

Last Wednesday, after Romney’s New Hampshire victory, the campaign’s top fundraisers met with campaign officials at his Boston headquarters for strategy briefings and a concerted push to bring new top donors on board.

“There were a lot of fired up people there who spent their own money to come to Boston because they’re excited about Governor Romney and they were making phone calls all across the country,” said Austin Barbour, a nephew of former Mississippi governor Haley Barbour and one of Romney’s national finance co-chairmen.

McCain, who attended the session, personally e-mailed and called many of his 2008 supporters and urged them to support Romney. “He urged everybody to get on board and to help,” Malek said. “He’s committed.”

Malek said he signed on with Romney in part because of Romney’s “inevitability.”

“I think people are starting to realize both the strength and inevitability of Romney as our nominee and are starting to come around,” Malek said.

One of them was Rep. Billy Long, a tea-party freshman from Missouri who recently endorsed Romney.

“He said to me, ‘I wasn’t sure, but I looked at how many ballots the others had missed and I thought, you know, if they can’t get on the ballot, how are they gonna get elected president?’ ” Talent said, relating his conversation with his fellow Missourian.

“I hear some people say, ‘You know, I’ve always liked Mitt, I’ve been thinking about him, I’ve got to do it,’ ” Talent said. “I hear other people saying, ‘You know, I wasn’t so sure at the beginning, I’ve been watching him, I think he can do it — and I know the others can’t.’ ”