Washington (CNN) -- Newt Gingrich might have led Republicans to their first House majority in 40 years in the 1990s, but the prospect of the former House Speaker becoming their presidential nominee is producing significant GOP angst.
"If he's the nominee, it's a disaster. There is no way to sugar-coat it," said one GOP congressional strategist describing the tension after Gingrich won South Carolina.
"There is a reason most people who know him best aren't supporting him," said a former House colleague still serving in Congress.
Asked why he had endorsed opponent Mitt Romney if he had served with Gingrich for so long, a House Republican replied with a smile, "Because I served with Gingrich for so long."
Those GOP congressional sources and many others spoke on condition of anonymity because they believe Gingrich, who is running against the "Republican establishment," will only turn the criticism into his advantage.
"The less they talk to the better," said one of the GOP sources concerned about Gingrich, "it only feeds into his narrative that the establishment is out to get him."
Gingrich's spokesman R.C. Hammond scoffed at the criticism, noting that the candidate himself said during CNN's debate last week what he regrets most in this campaign is wasting time attacking anyone besides President Barack Obama.
Veteran Republican leadership aide Ron Bonjean said on the record what most of his colleagues would only tell CNN privately.
"Most people on Capitol Hill and in Washington are very nervous about a Gingrich candidacy," he said. "It sends a shiver down a lot of Republican spines."
"You can actually feel the nervousness from Republicans around town that Gingrich could actually bring the craziness back of his speakership from the 1990s. It's everywhere."
Several GOP congressional sources told CNN that it's not only about Gingrich getting to the White House -- Republican leaders worry that Gingrich at the top of the ticket could be a drag on their candidates for Congress and even hurt chances for taking over the Senate. They point to races in Virginia, Ohio, Florida and Nevada as examples of Senate races that could be in jeopardy.
Why? It's mostly Gingrich's reputation as a chaotic campaigner and undisciplined messenger.
The most frequent example: when he called the House Medicare reform plan most Republicans supported "right wing social-engineering."
To be sure, some, like Sen. Jim DeMint, R-South Carolina, who has not endorsed a presidential candidate, dismiss concern about Gingrich.
"I think what we've seen from Newt that people like is willingness to take on the media and to really stand up and fight," DeMint said.
Still, just 12 sitting Republican lawmakers have endorsed the former speaker, while 64 support Romney.
And many former Gingrich congressional colleagues have endorsed Romney. As Gingrich's support began to climb, Susan Molinari even made a Web ad in which she said, "I served with Newt Gingrich in Congress. Newt Gingrich had a leadership style that can only be described as leadership by chaos."
The Gingrich campaign e-mailed a list of 11 former members of Congress who served with Gingrich who are backing him now.
"We had a balanced budget with Newt, first time, only time in my lifetime we didn't spend out more money than we took in," argued J.C. Watts, one of the 11.
Most GOP congressional sources who tell CNN that Republican leadership is concerned about Gingrich also say they doubt he will eventually get the nomination.
"Most members don't expect him to go further," said a senior GOP leadership aide, adding that they don't want to "waste bullets attacking the guy not going anywhere anyway,"
But members are watching Florida very carefully.
"We are not at Defcon 5 yet, but we'll see what happens in Florida," said another one of the worried GOP strategists.
If Gingrich does win, veteran GOP strategists tell CNN to expect pressure on Senate Minority Leaders Mitch McConnell, House Speaker John Boehner and other Republican leaders to call key GOP donors and ask them not to contribute to Gingrich's campaign.
CNN Chief National Correspondent John King contributed to this report.