American Legacy PAC, founded by conservative businessman Mike Murray, counts Gingrich and his wife as honorary co-chairs. It's already contributed $5,000 to Gomez's campaign.

Murray said the group plans to raise $1 million in the next three months, largely through small-dollar contributions from grassroots donors, and could contribute more to Gomez going forward. He said that if beleaguered Rep.-elect Mark Sanford could close a double-digit deficit in the polls in the final weeks of his South Carolina special House election, Gomez has a chance to win, too.

"We certainly think he's electable, and looking at what Mark Sanford was able to do in terms of closing the gap, we think that's possible here, as well," he said.

In an email sent to supporters on Wednesday, Gingrich highlighted the race, saying that Democratic candidate Rep. Edward MarkeyEdward (Ed) John MarkeyRegulators seek to remove barriers to electric grid storage Markey, Paul want to know if new rules are helping opioid treatment Oil spill tax on oil companies reinstated as part of budget deal MORE "clearly is rattled" by a recent poll showing Gomez lagging him by just 4 percentage points.

He criticizes a controversial ad from Markey's campaign that shows Gomez's face next to Osama bin Laden's, saying it's "guaranteed to backfire."

"Why a liberal anti-war Democrat would want to attack a Navy SEAL with that kind of over the top ad is beyond comprehension," he says.

That ad has drawn fire from Republicans, and Republican National Committee Chairman Reince Priebus called for Markey to take it down.

But support from outside groups like American Legacy PAC will stoke Markey's main line of attack, on Gomez's refusal to sign a pledge to keep outside money out of the race.

He's attempted to tie Gomez to shadowy special interest groups and national Republican figures like the Koch brothers for his refusal to sign the pledge. Gingrich's backing could also offer Markey another opportunity to tie Gomez to the national GOP, an argument that could be devastating to Gomez, as Massachusetts voted more than 60 percent for President Obama in 2012.

And a poll released Wednesday indicates more than 70 percent of voters believe the pledge is an important issue in the race.

However, a Boston Globe report on Thursday could complicate Markey's attacks, as it revealed that between 2003 and 2012 Markey's received $2.7 million from PACs, about a third of the total donations he received over that period.

--This post was updated at 6:50 p.m. to reflect comment from Murray.