The primary battle between two Bucks County Democrats vying in one of the nation's most-contested congressional races is heating up, with both candidates trading accusations.

A supporter of State Rep. Steve Santarsiero threatened Monday to file a Federal Election Commission complaint against Shaughnessy Naughton, Santarsiero's primary opponent. Meanwhile, Naughton's campaign has hammered the other side over taking credit for gun legislation that never became law.

After an independent science-related political action committee founded by Naughton sent a mailer blasting Santarsiero, a board member of the PAC complained Monday. The member's letter to the FEC alleges that the Naughton campaign unlawfully coordinated with the group, 314 PAC, which her office denied Monday.

Naughton's campaign said there was no violation of federal law.

The race for the Eighth District seat has drawn national attention. The district covers Bucks County and a small part of Montgomery County. Rep. Mike Fitzpatrick, a Republican, is retiring, and with no incumbent and an electorate nearly evenly split between Republicans and Democrats, the seat is viewed as up for grabs.

On the GOP side, Fitzpatrick's brother, Brian, is running against former County Commissioner Andy Warren and Marc Duome, a neuropsychologist.

Monday's questions about the PACs are "nothing more than a political stunt by the [Santarsiero] campaign," Naughton's campaign manager, Erik Polyak, said.

Still, there have been close ties between 314 PAC and Friends of Shaughnessy Naughton.

According to FEC filings for each group, both were paying Josh Morrow, an adviser to Naughton's campaign, last year.

The most recent filings available show that Morrow collected $20,000 from 314 PAC between July and December 2015 for consulting work. He also was paid a total of $12,000 in October and December 2015 by Friends of Shaughnessy Naughton, according to the filings.

Morrow no longer works for 314 PAC, Naughton's campaign said.

Det Ansinn, the 314 PAC board member who said he planned to file the FEC complaint Monday, said he was "shocked" that the mailer was "put out by a PAC that I'm associated with."

Ansinn supports Santarsiero. He said he and a few other board members with whom he had spoken were not aware of the mailer before it went out.

"It certainly doesn't look good," said Eric Goldman, Santarsiero's spokesman.

The Santarsiero campaign has been ribbed by both Naughton and Republicans for claiming that he wrote Pennsylvania's gun safety law, when the bill he proposed was never passed.

None of the 54 bills for which Santarsiero was the primary sponsor during his time in the legislature has been signed into law, although dozens of bills he co-sponsored have passed and his ethics bill was adopted as a rule in Harrisburg.

"There's more to being a legislator than just passing bills," Goldman said.

Naughton's campaign called the FEC complaint a distraction.

"If we can't believe what Steve says about his own record, why should we believe what he says about anyone else's?" Polyak said.

jmcdaniel@philly.com