SALEM — US Representative John F. Tierney always knew he could face a tough rematch against his old nemesis, Republican Richard R. Tisei, in the November election. But now there are signs that he is in a potentially tight race to win the Democratic primary on Tuesday.
In recent days, Tierney and his Sixth District challenger, Seth Moulton, have been exchanging increasingly bitter attacks — a sign, in the absence of public polling, that Tierney may be worried, political observers say.
On Tuesday, it was Tierney launching a blistering attack ad that attempts to tie Moulton to Republicans who support the National Rifle Association and oppose abortion rights. On Wednesday, it was Moulton’s turn to respond.
Standing in Salem Common with six supporters, the Marine veteran demanded that Tierney renounce the ad, which could be deeply damaging to him in a primary dominated by loyal Democrats.
“It’s an absurd attack from a desperate politician,” Moulton said. “The fact of the matter is, I have been a Democrat my entire life.”
Observers said Tierney’s decision to strike Moulton with a negative ad suggests the race may be tightening.
“I don’t think there’s any question it’s competitive,” said Nathan Gonzales, deputy editor of the Rothenberg Political Report, a nonpartisan newsletter that tracks political races nationwide. “But with a lack of data, it’s not clear how close it is.”
Gonzales pointed out that Moulton has raised significant sums — more than $1.6 million, compared with Tierney’s $1.9 million — “and has to be taken seriously.”
“And by Tierney’s response, and now his ad, it’s clear that he’s not taking his renomination for granted,” he said.
Tierney insisted Wednesday that he was not worried. He said he had to hit back at Moulton after his foe released a negative ad last month accusing him of passing just one bill during his 18 years in Congress, and of missing more votes than most other members.
“We had people stopping us every day, saying, ‘Why don’t you tell us who he is, and where he gets his money? And how come you’re going to let him lie in an ad?’ ” Tierney said after addressing a group of retired federal employees at a senior center in Peabody. “We’re not going to take anything like that lying down.”
Contrary to the accusations in Moulton’s ad, Tierney said he has a 97.5 percent lifetime voting record and has written several measures that have become law. But those measures were folded into larger bills so they didn’t carry his name atop the final legislation, he said.
“It’s either an incredible lack of knowledge of how Congress works, or he’s just being cynical,” Tierney said.
Tierney, a nine-term Salem Democrat, is considered vulnerable after narrowly surviving a bruising reelection fight against Tisei in 2012. That race was dominated by debate over a gambling scandal involving Tierney’s family.
In 2011, Tierney’s wife, Patrice, served a month in federal prison after she acknowledged helping her brother file false tax returns in connection with his illegal offshore betting operation.
The acrimonious contest this summer has clearly divided voters in the district, which ranges from the North Shore to the Boston suburbs of Burlington and Bedford. Tierney’s supporters argue he has delivered for constituents, while detractors say he is part of a gridlocked system.
Mike DiPietro, a retired inspector at the General Electric Plant in Lynn, said he supports Tierney, whom he called a longtime defender of the plant. But he said he wouldn’t be surprised if he lost on Tuesday.
“I think the people are just plain tired of what’s going on in Washington,” he said. “They’re doing nothing. And that’s not good. ”
Tierney’s ad seizes on a $1,000 donation that Moulton received from the White Mountain PAC, a federal political action committee affiliated with former US senator Judd Gregg, a New Hampshire Republican.
Over a black-and-white photo of Moulton and foreboding music, a narrator says: “Moulton took money from a special interest group that only funds Republicans. NRA-backed Republicans who voted to outlaw abortions. Tea Partiers who say they’ll end the Medicare guarantee.”
The narrator adds that Moulton’s “special interest friends” gave money to 67 Republicans, but just one “politician who wants you to believe he’s a Democrat.”
Moulton said that he had returned the unsolicited donation from the PAC because he does not share the views of the Republicans the group supports.
Gregg said he donated to Moulton at the urging of his son, Josh, who has been a friend of Moulton’s since the two were students together at Harvard Business School.
“My son said, ‘This a guy I like and admire,’ ” Gregg said. “And I said, ‘If you like him and admire him, I’d be happy to help him out.’ ”
He lamented that Tierney is now using the donation to attack Moulton, calling it a sign of partisanship.
“I just think people who are shouters from the corner and aren’t willing to cross the aisle aren’t going to be effective players in Washington,” Gregg said.
Marisa DeFranco, a Middleton immigration lawyer who is also running in the Democratic primary, said she was content to let her foes bicker.
“They can fight it out while I talk to voters,” she said.
Michael Levenson can be reached at mlevenson@globe.