Mitt Romney blasted Republican presidential rival Newt Gingrich on Tuesday as an “extremely unreliable leader in the conservative world” who has taken positions in the campaign that should give GOP voters pause as they consider their choices for the party’s nomination.

Romney, in an interview with The Washington Post, offered some of his toughest criticism to date of the politician whose sudden rise in the polls has made him, at least for now, the front-runner for the nomination. He also had tough words for President Obama and his campaign, saying he would not let them portray him as a tool of Wall Street and calling the president “a member of the 1 percent.”

But Romney also acknowledged that he has not always been in step with conservatives and said that he was “wrong” not to sign Gingrich’s 1994 Contract with America when he was running for a Senate seat in Massachusetts.

Romney’s effort to portray Gingrich as unreliably conservative signals a new and sharper strategy to blunt the former speaker’s momentum and to raise doubts about him in the eyes of voters who may be examining him closely for the first time in the campaign.

The former Massachusetts governor once was seen as at least a nominal front-runner in the Republican race but now finds himself trailing Gingrich in a number of states with early contests, as well as nationally.

Defending himself against charges that his own conservative credentials are suspect, Romney turned the question in Gingrich’s direction and said that it is the former House speaker who has strayed repeatedly from embracing conservative doctrine in recent years.

“He has been an extraordinarily unreliable leader in the conservative world — not 16 or 17 years ago but in the last two to three years,” Romney said. “And even during the campaign, the number of times he has moved from one spot to another has been remarkable. I think he’s shown a level of unreliability as a conservative leader today.”

Romney repeatedly cited what Gingrich has said about the Medicare reform proposal offered by House Budget Committee Chairman Paul Ryan. Earlier this year Gingrich dismissed it as “right-wing social engineering,” and in an interview released Tuesday he called the plan political “suicide,” given its limited public support.

Romney also opened a new line of attack by personally criticizing Gingrich for having made a television commercial in 2008 with Rep. Nancy Pelosi (D-Calif.), then the House speaker and now the minority leader, promoting a campaign against global warming.

“Let’s look at the record,” he said. “When Republicans were fighting for cap and trade and needed a leader to stand up against cap and trade, he did an ad with Nancy Pelosi about global warming. When Republicans took one of the most courageous votes I’ve seen in at least a decade to call for the reform of Medicare under the Paul Ryan plan, he goes public and says this is a ‘right-wing social engineering’ plan. Even today he called it ‘suicide.’ ”

Romney’s campaign plans to escalate the attack on Gingrich over his work with Pelosi on climate issues with a video that will be released Wednesday, a campaign official said.

The 35-minute interview came as Romney was preparing for Thursday’s debate in Iowa, a crucial follow-on to the forum held Saturday night in Des Moines. The debate will be the last before the Iowa caucuses on Jan. 3. Romney shucked his suit coat for the interview, appearing relaxed in a white shirt and blue tie.

Romney acknowledged that in 1994, when Gingrich was leading the campaign that resulted in a Republican takeover of the House and Senate and he was running a losing race for the Senate seat held by Edward M. Kennedy (Mass.), the two were at odds, with Romney out of step with conservative doctrine and strategy.

Romney declined to sign the Contract With America, the Gingrich-led campaign manifesto for GOP candidates that fall. In a debate with Kennedy in the fall of 1994, Romney also distanced himself from the administration of President Ronald Reagan, claiming “I was an independent” during those years.

“I applaud the fact that he was wise in crafting the Contract With America,” Romney said of Gingrich on Tuesday. “I didn’t think it was a very good political step. He was right; I was wrong.”

He also said his “admiration and respect for the policies of Ronald Reagan has grown deeper and deeper” over time.

But he acknowledged that some Republicans question whether he is authentically conservative and said he must do a better job to convince them that he is.

“There are some elements that create the impression that I may not be a conservative,” Romney said. “One is being from Massachusetts. The other is a health-care plan that people feel was in some ways a model for what Barack Obama did. . . . People, I think, question those conservative values, and I have to bring them back to my record and, frankly, my writings.”

With Gingrich surging, Romney was asked to describe his path to the nomination. “Eleven hundred and fifty delegates, approximately,” he replied with a laugh. Asked whether he had to win Iowa or New Hampshire or other early states, he demurred. “I don’t have to win anything other than 1,150 delegates.”

He said that he would be surprised if he did not win Massachusetts and Utah and that he hopes to win New Hampshire. “Right now I’ve got a good lead in New Hampshire — I hope to maintain a lead there and to win,” he said. “But I think it’ll be a close race in New Hampshire.”

Romney sketched out elements that could prolong the race. He said he was not certain that lack of money would knock candidates out of the race this year, even if they sustained early losses. “I think it’s also possible that this will go on for a longer period of time than prior Republican primaries, in part because of the rule change toward proportional awards of delegates,” he said.

An NBC News-Wall Street Journal poll released Tuesday night showed Gingrich with the highest percentage of any Republican to date but with potential problems in a general election. Romney acknowledged that Gingrich is currently the front-runner, but he predicted that his rival could face serious scrutiny from voters who have been looking for an ideal candidate and said that could affect his standing with the voters.

“Speaker Gingrich was at single digits for a long time, suddenly jumps to a lead — that doesn’t suggest a very thorough examination led to that level of support, rather an expectation of an ideal candidate, and he may or may not be able to fulfill that expectation,” he said.

Romney also addressed what many analysts consider a clear vulnerability: the fact that he has not been able to expand his support in the national polls much above 25 percent. He put a positive gloss on the numbers. “I’ve been either in the lead or second in the rankings pretty much all year, and that suggests a level of stability that I hope is able to be built upon in the contests themselves,” he said.

At times, Romney appeared to pull his punches with Gingrich. He was asked about a moment in Saturday’s debate during a discussion of the former speaker’s characterization of the Palestinians as an “invented” people. Romney said that night that he was not a bomb thrower “rhetorically or literally.”

“I wasn’t referring to the speaker,” he said. “I was referring to myself. I said I was not a bomb thrower. I don’t think I would’ve said that he is a bomb thrower. I don’t think I would’ve said that. What I would’ve said is that I am a person of sobriety, steadiness, patience, and act in a deliberate way to protect America’s interests and that of our allies.”

But he quickly returned to Ryan’s Medicare proposal. He said entitlement reform is “one of the most defining issues of conservatism today” and attacked Gingrich again on the subject.

“I think his comment on the Paul Ryan plan was an intemperate comment. I think him calling it suicide is a mistake. I know it can be popular with some people to use extreme language, but we’re talking about the presidency of the United States, and if one says intemperate things in a campaign, might one say the same kinds of things in the White House? And the consequences are potentially severe.”

As the interview concluded, Romney addressed the general election and attacks that have begun from the Obama team. He said he believed he would be the strongest candidate and offered a preview of how he would defend himself.

“I believe the question on the minds of the American people will be whether or not President Obama’s policies have failed and are likely to continue failing them,” he said. “I do not believe class warfare and demonization of an individual or a percentage of Americans is the course for building a greater America. This is a president who talked about bringing America together. Is he going to run a campaign based on tearing America apart?”

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