“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.”…

“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.