Republicans on Capitol Hill are increasingly worried about the potential of a Newt Gingrich presidential nomination, fearful that the former House speaker’s reputation for volatility could drive away independent voters and jeopardize GOP prospects for controlling Congress in 2013.

Just a few months ago, Gingrich was an afterthought in the campaign. But his sudden rise over the past month has given pause to congressional Republicans who thought President Obama’s troubles with the economy could firm up their majority in the House and a new majority in the Senate.

The resistance to Gingrich’s surge is based on his reputation for unpredictabilty and a recognition that the success of Republican congressional campaigns will be inextricably linked to the party’s nominee.

One GOP strategist, Mike Murphy, this week described the potential of a Gingrich nomination as a “train wreck,” while Tom Davis, a former House campaign chairman, worries that Gingrich at the top of the ticket could be a drag on the party’s congressional chances, particularly in the Northeast.

Gingrich’s fiery speeches have energized some conservative voters on the campaign trail. But his tumultuous tenure in Congress has many Republicans fearing that his presidential campaign could jolt House and Senate candidates off their message of shrinking federal spending and blaming Obama for an economic malaise.

“The formulation of your message and the discipline with which you deliver it becomes much more important when you’re on a national stage every day,” said Sen. John Thune (S.D.), the newly elected message chief for the Senate GOP and a backer of Gingrich’s top rival, former Massachusetts governor Mitt Romney. “I do think that it’s going to be critical to our overall effort, if we’re going to be successful in both House and Senate campaigns next year, to be able to have a coherent message and one which is very much synced up with our nominee.”

With the economic recovery struggling to take hold, House Republicans had hoped to consolidate their gains from the 2010 midterm elections, aiming to return in 2013 with a conference close to its current size of 242. Senate Republicans, needing just four seats to claim the majority, are in a strong position because Democrats are defending 23 seats and have several key incumbents retiring. Republicans are defending only 10 seats.

“He’s an idea guy, and a lot of his ideas are good and some of them are wacko. So he’s the kind of guy that needs to be involved somehow — I just don’t think he needs to be the leader,” said Rep. Mike Simpson (R-Idaho), a seven-term congressman who recently endorsed Romney.

According to data compiled by Roll Call, Romney has secured 56 endorsements from congressional Republicans, while Gingrich has eight.

House Speaker John A. Boehner (Ohio), who was one of Gingrich’s lieutenants after the GOP won control of the House in 1994, would say only that Gingrich is “a friend.”

At a policy forum Wednesday sponsored by Politico, Boehner stressed differences between his background as a small-business man and Gingrich’s as a college professor.

“Like all big thinkers, they have got some great ideas, then they have some other ideas,” Boehner said, echoing a frequent criticism that Gingrich too often hopscotched from one incomplete idea to the next.

Some leading Democrats share the thinking that a Gingrich nomination could help them with independent voters, in both the presidential and congressional elections.

“Newt Gingrich leading the Republican ticket brings those independent voters back to Democrats,” said Rep. Steve Israel (N.Y.), chairman of the Democratic Congressional Campaign Committee. “He is the king of gridlock. These are the very independent voters who had it with him in 1995 when he tried to shut down the the government.”

Not all Republicans agree that Gingrich would be political dynamite for down-ballot contests. Some simply cannot decide how he would play in a general election.

“If this becomes a race between competing visions of America, and not about personalities, I can’t think of a better spokesman than Newt Gingrich getting up there and being able to articulate a competing vision,” said Davis, a former congressman from Virginia who was first elected in 1994 as part of the Gingrich revolution. But Davis warned that Gingrich’s past ethics issues and his extramarital affairs provide Democrats with ammunition against GOP candidates.

“If this becomes about all the other baggage and so on, he’s a loser,” said Davis, who twice chaired the National Republican Campaign Committee, the House GOP’s campaign operation. “The question is: Can you get there or can you not get past [the baggage]? I think that the jury is still out on that.”

Many of the junior conservative Republicans who never served with Gingrich are confused, and they would now prefer that the race go on long enough to reveal which of the top candidates has the greater vulnerabilities.

“The longer that this goes on, the better, to really flesh out where these guys are going to be on issues and see the team they’re going to put around them,” said Rep. Devin Nunes (R-Calif.), who was elected in 2002. “I’m actually hoping this goes all the way to California.”

The thought of such an extended primary campaign — the Golden State GOP primary is slated for June 5 — scares many Republican strategists, who prefer that a nominee emerge by early spring to begin raising the hundreds of millions of dollars needed for a general-election campaign.

Topping the list of Gingrich’s detractors are many of these senior GOP strategists who have run the House and Senate campaigns for the past decade, along with senior lawmakers who recall Gingrich’s rocky tenure as House speaker. These Republicans heard Gingrich’s recent claim that the Palestinians were an “invented people” and recalled the many times GOP lawmakers were called on to defend controversial statements Gingrich made during his four years as speaker.

“This is ‘Newtonium.’ Newt Gingrich is radioactive material. The establishment thinks if they get too close, he could kill every Republican on the ballot,” pollster Alex Castellanos said Sunday on NBC’s “Meet the Press.”

By the time Gingrich announced his retirement from the House in November 1998, he had become a polarizing figure, with a favorability rating of 34 percent, according to an ABC News poll at the time.

He had become a political liability, particularly in the Northeast. In the 1996 and 1998 elections, House Republicans lost more than half a dozen seats in that region, beginning a long slide that culminated with the loss of the House in 2006 and and no House Republican from New England after the 2008 elections.

In 2010, however, the party bounced back and picked up 15 House seats in the Northeast — seats that some see as being in danger with Gingrich as the GOP standard-bearer.

“It’s tougher. You want Romney, period,” Davis said. “He plays more in the Northeast. I don’t think there’s any question about that.”