Officials at the Democratic Congressional Campaign Committee say they do not comment on internal polling data, but sources say the national campaign arm of House Democrats has polled voters in Connecticut’s 5th District — and Dan Roberti is in the lead.

Roberti, 30, of Kent, will face former state Rep. Elizabeth Esty and House Speaker Chris Donovan next week in the Democratic primary. Donovan, the party endorsed candidate, was once considered the frontrunner. But his campaign was turned on its head by an ongoing federal investigation that has seen the arrest of two of his top campaign staffers and six other state residents.

Josh Nassi, Donovan’s former campaign manager, and Robert Braddock, his former finance director, have both been charged with conspiring to hide the source of nearly $30,000 in campaign donations. The federal indictment alleges that roll-your-own tobacco shop owners used straw donors to make campaign donations while Donovan was serving as Speaker of the House in order to defeat legislation detrimental to their business interests.

Donovan has repeatedly said he had no knowledge of the scheme. But news coverage of the investigation and attack ads aired by the other two candidates apparently have taken a toll on the candidate. Sources with knowledge of the poll say Roberti has taken the lead.

Roberti had been one of the most outspoken critics of Donovan’s. He’s repeatedly called upon the speaker to drop out of the race because of the investigation. But rhetoric from the Esty and Roberti campaigns shifted recently, going from attacking Donovan in TV spots and press releases to criticizing one another instead.

Toward the end of a largely tame debate Saturday, Esty found an opportunity to jab Roberti for being, until recently, co-owner of his father’s Washington lobby firm. She’s also aired commercials calling Roberti a co-owner of the lobbying firm and alleging that special interests connected to his father, Vin Roberti, have bankrolled his campaign.

On Tuesday Roberti aired his own ad, in which he looks at the camera and says, “I’ve never been a lobbyist and Elizabeth knows it.”

“Elizabeth Esty’s attack ad and her collecting money from polluters her husband regulates is everything that’s wrong with politics,” Roberti says, referring to Esty’s husband, Dan Esty, who heads up the state Department of Energy and Environmental Protection.

Little is known about the DCCC poll, other than that it puts Roberti ahead of Esty and Donovan. Stephen Carter, a regional spokesman for the committee, would not comment on its existence.

But a Roberti win next week would be something of an upset in Connecticut, where he was virtually unknown until recently. Roberti has never held an elected office.

Roberti does have some well-connected friends, however. James “the Ragin’ Cajun” Carville, a well known Democratic political strategist who helped elect former President Bill Clinton, sent out a fundraising letter for Roberti when he entered the race last April.

Roberti’s also worked with acclaimed documentary filmmaker Ken Burns on a national parks project. And last week, Martin Luther King III offered support for Roberti.

“I have to say that when we look at the kind of people that we want to represent us, certainly Dan Roberti is one of those kind of people that we need in Congress,” the Register Citizen reported King saying.

Both the Esty and Roberti campaigns have been tight-lipped about their own internal polling data and neither side wanted to comment on the DCCC poll. The race is believed to be very close with just seven days until the primary.

And primary polling can be notoriously unreliable. Roy Occhiogrosso, a longtime Democratic operative and Gov. Dannel P. Malloy’s senior communication adviser, said that’s because pollsters are attempting to get a sample of such a small group of people.

“It’s very difficult for public polling groups to capture a representative sample of the universe of people that will actually come out and vote on primary day because the universe is so small,” he said.

Occhiogrosso said that in the week leading up to the 2010 primary in which Malloy defeated primary challenger Ned Lamont, not a single poll showed Malloy ahead. But he went on to win by 16 points.

Though he said trying to predict the result of an election on public polling is an “iffy proposition,” Occhiogrosso said they are useful tools gaging trends and momentum in candidates.