Newt Gingrich, left, and Mitt Romney cross paths at a debate in November. (JONATHAN ERNST/REUTERS)

Republican candidate Mitt Romney on Thursday launched his first sharp attack on rival Newt Gingrich — choosing to focus on Gingrich’s leadership as House speaker and using old colleagues to paint him as vain, erratic and unreliable.

The effort began in earnest with a morning conference call with the news media, in which James M. Talent, a former congressman and senator from Missouri, and John Sununu, who had been chief of staff to President George H.W. Bush, ripped into Gingrich.

“What we’re here to say — with reluctance, but clearly — is that he’s not a reliable and trusted conservative leader,” Talent said. “Because he’s not a reliable or trustworthy leader.”

Gingrich “is more concerned about Newt Gingrich than he is about conservative principle,” Sununu said.

The new approach marks a clear turn in a campaign that has been notable so far for a lack of one-on-one combat. It was also a signal that Romney feels threatened by Gingrich’s sudden lead in three of the four early states and aims to aggressively take him on as the pair head for a series of January showdowns, starting with the Iowa caucuses in less than four weeks.

Gingrich, who said on Thursday that he would “stay positive,” is aiming to use the next few weeks to remind voters that he’s the same leader whose big ideas and sharp rhetoric inspired a Republican “revolution.”

Romney will try to make certain voters remember that Gingrich is the same leader who had to leave that revolution in mid-stream, resigning in 1998 after less than four years as speaker amid ethics charges and dissension from GOP lawmakers.

Gingrich “got a plane that hadn’t flown in 40 years to fly,” said former congressman Christopher Shays (R-Conn.), who supports Romney. “But sometimes we went to the left. Sometimes we went to the right. Sometimes we went straight up. Sometimes we went straight down.”

“Newt is an entrepreneur more than he’s a manager,” Shays said.

Romney could have hit Gingrich in other ways. The candidate’s biography includes three wives, a $300,000 fine for ethics violations and a $250,000-plus charge account at Tiffany. Gingrich has changed his tune on subjects from health-care reform to climate change — and he recently scoffed at the entire notion of child-labor laws.

But Romney hopes to reverse a slide by turning one of Gingrich’s biggest assets — his experience in Washington — into a weakness.

The campaign hopes to paint Gingrich as a philosopher-politician whose inconsistent behavior has undercut the conservative agenda. In contrast, Romney’s campaign will stress its own candidate’s years as a businessman working in the real world.

“Gingrich creates theories,” their message goes. “Mitt creates jobs.”

The Romney campaign provided talking points to its congressional supporters that stressed this contrast: “Gingrich has spent a lifetime operating in theory while Mitt has succeeded in practice.”

Asked about Romney’s attacks on Thursday, Gingrich responded with the same combination of charm and ego that distinguished him in the 1990s. He dismissed the charges, and the candidate himself, at the same time.

“We’re going to stay positive, we’re going to stay solution-oriented and talk about what America needs to do,” Gingrich said after a campaign stop in Greenville, S.C. “The only opponent I have is Barack Obama.”

Even before Romney’s attacks, Gingrich’s chaotic, historic years as speaker had already become a focal point in the campaign.

In interviews, his former colleagues have said that Gingrich was a brilliant insurgent — energizing a Republican caucus that had not held power in 40 years, and winning a historic victory in the 1994 elections.

After taking office, Gingrich led the House into a series of battles with then-President Bill Clinton over deficits and spending. One standoff, in the winter of 1995-96, led to a temporary shutdown of the federal government.

But several colleagues said the fights were worth it. Republicans under Gingrich won major changes to the welfare program, as well as a budget agreement with Clinton that led to four years of balanced budgets.

“This is the single most successful speakership in modern American history,” former congressman Robert Walker (R-Pa.) said of Gingrich on Thursday. He called Romney’s attacks “nonsense.”

Several former colleagues said that Gingrich’s personality — full of rapid-fire ideas but sometimes short on personal skills — might actually be well suited for a tumultuous time in Washington.

“Everybody in politics enjoys peace and quiet,” said Rep. Jack Kingston (R-Ga.), a Gingrich supporter. “But we really weren’t sent here for peace and quiet. We were sent here to get things done. It would certainly be putting the vehicle of government in a different gear” if Gingrich were elected, he said.

But other colleagues remembered Gingrich as a deeply flawed boss. Former congresswoman Susan Molinari (R-N.Y.) said that Gingrich had begun as an inclusive leader, sharing credit and public appearances with his rank and file.

Slowly, however, Gingrich began to portray the GOP revolution as “All Newt, all the time,” Molinari said. She said Gingrich often issued conflicting marching orders from day to day, and focused on a decades-long plan for GOP dominance, ignoring real problems in the present. “That’s when his leadership style really sort of evolved into leadership by chaos.”

Gingrich’s time as speaker was marred by a series of public gaffes. He intimated that the government shutdown was, in part, the result of a personal snub by Clinton: Gingrich had been made to sit in the back when he flew once on Air Force One.

Eventually, Gingrich had a press aide stand in his field of vision when he spoke to reporters — nodding, or shaking her head, to signal if he was in trouble.

Gingrich’s high-handed style led to an abortive effort to oust him as speaker, in 1997. Gingrich survived that, but after a disappointing election cycle, he resigned in 1998.

“It’s amazing to me” that Gingrich is back as a serious presidential candidate, Molinari said. Her husband, former congressman Bill Paxon (R-N.Y.), was a player in the abortive coup, and resigned his position in the GOP leadership. “He’s the guy that got thrown out. I mean, people just don’t lose their speakership.”

Do Gingrich’s troubles as speaker mean that he’d be a bad president? Former congressman Zach Wamp (R-Tenn.) said he wasn’t sure. In some ways, he said, Gingrich might actually be more suited to the White House than the House — after all, the speaker can’t fire lawmakers who disagree with him.

“He’s not a real relational guy. He’s not a back-patting politician,” said Wamp, who has not endorsed a candidate. “That was harder for him as speaker than it would be as president, because as president you lay out the vision” and others carry it out.

So a combative, ambitious, ego-driven President Gingrich might succeed, Wamp said.

At least for one term.

“He would actually get something done,” Wamp said. “Even if he never got reelected.”

Staff writer Nia-Malika Henderson and staff researcher Alice Crites contributed to this report.