Sen. John Thune’s (S.D.) endorsement of Mitt Romney leaves little doubt Congressional Republicans consider the former Massachusetts governor the most electable GOP presidential candidate, and it signals fears that former Speaker Newt Gingrich (Ga.) could hamper the ticket and cost Republicans the Senate majority as the nominee.

Thune’s embracing of Romney last week in Iowa might also reveal the South Dakotan’s lingering ambition for higher office. Thune, who pondered a 2012 White House bid of his own, declined to rule out accepting an invitation to serve as Romney’s running mate, or in his Cabinet, should either scenario materialize. But he said such opportunities were irrelevant to his endorsement.

Thune’s emphasis on Romney’s electability against President Barack Obama — even as he vouched for the former governor’s conservative credentials — appears to betray anxiety over the abilities of the rest of the field in a general election.

Republicans also worry that a weak nominee could diminish the party’s solid prospects for flipping the four seats needed to retake the Senate.

“I’ve looked at it, watched the debates, came to the conclusion that both in terms of what we need as far as experience and skill set, he’s got the right leadership abilities and experience to try and turn the country around, and secondly, I think he’s electable,” Thune told Roll Call in an interview. “The goal is to make sure that we keep the current president from getting a second term, and I think [Romney] is best positioned to do that.”

Thune, the Senate Republican Policy Committee chairman, is set for a promotion to Conference chairman in January when Sen. Lamar Alexander (Tenn.) will voluntarily relinquish that post. Thune has not ruled out running for Whip, the No. 2 party leadership position, in the next Congress.

The Romney endorsement came just as Gingrich began rising in national and early caucus and primary state polls, positioning the former Speaker as the latest Republican to audition for the role of consensus alternative to Romney, who is considered too moderate and philosophically malleable in some conservative circles.

The upside for Romney is obvious, as much as the backing of a Capitol Hill figure might be helpful when Congress’ approval ratings are near single digits. It could lend him badly needed conservative credibility. Sen. Roy Blunt (Mo.), who is leading Romney’s Member-whip operation, said Thune’s “values and world view in South Dakota are deeply understood in Iowa. … There was a reason they made that endorsement in Des Moines, not in Sioux Falls.”

Republican operatives also see potential advantage for Thune, saying that the pact is mutually beneficial.

“He is clearly positioning himself early for vice president or a Cabinet-level position,” said one GOP strategist with relationships in the Senate. “He knows that Romney needs someone like Thune who has strong conservative credentials, a respected member of Senate leadership and effective spokesman on the circuit.” 

“Those are all hypotheticals,” Thune said. “I made a decision to do this not predicated upon any of that. But I certainly want to be helpful to the degree that I can to [Romney], and to his team, because I concluded that he’s the right person.”

The RealClearPolitics.com average of polls taken Nov. 8 through Tuesday gauging an Obama-Gingrich contest gave the president a lead of 48.7 percent to 43 percent. By comparison, Obama led Romney 45.9 percent to 44.4 percent, and the former governor has bested Obama in several polls taken this fall.

Republicans, while lauding Gingrich’s depth of knowledge and role as a GOP leader over the course of his 33 years in public life, including a two-decade House career, have serious concerns about his viability in a general election against Obama.

One GOP lobbyist described Gingrich as a “target-rich environment for a negative campaign,” echoing concerns on the Hill that the former Speaker might drag down the ticket.

Gingrich’s past includes personal foibles and professional hiccups. He is on his third marriage and it is widely known that the first two ended after bouts of infidelity. His Speakership, while historic and marked with major conservative policy achievements, ended after he lost a battle over government spending with President Bill Clinton, whose party gained five seats in the 1998 midterms despite the impeachment scandal.

Gingrich also was the subject of Congressional ethics investigations.

After retiring, he conducted work that has been referred to by some as lobbying, while also displaying philosophical flexibility. He taped an ad with then-Speaker Nancy Pelosi (D-Calif.) about climate change and at one point expressed support for a health insurance mandate — an issue that has dogged
Romney. Gingrich has said on the campaign trail he regrets the Pelosi stunt.

Republicans also worry that he is simply not likable, particularly among women, an often-important attribute in presidential elections.

“A Gingrich nomination would hurt our efforts to take the Senate and hold the House. Obama has $1 billion to savage our nominee,” said the Republican lobbyist, who has Senate ties.

The lobbyist believes Romney would help the GOP win the Senate majority. “Romney may not be a god, but he is not harmful,” the lobbyist said.

“The problem with Newt on the top of the ticket is that he’s the past. Obama has already proven he can beat the past,” added a GOP strategist with Senate clients. “We need a nominee who can win the present and the future. I’m not sure Newt ever gets to have a debate about the future.”

National Republican Senatorial Committee Chairman John Cornyn (Texas)  declined to comment directly on the GOP presidential primary contest. But he said it is important for his party to nominate an individual who proves that he or she can beat Obama next November. Cornyn said he expects Republican primary voters to unify behind just such a candidate.

“Certainly the better our nominee does, particularly in a lot of these states like Florida, Missouri, Pennsylvania, Ohio, where we picked up seats in 2010 — that will certainly help quite a bit,” he said. “I think electability is quickly becoming the test.”