Rep. John F. Tierney faces the toughest primary of his 18-year congressional career Tuesday, when he will attempt to stave off a well-funded, top-flight Democratic opponent.  

For months, polls have shown Tierney with a hefty advantage over Iraq War veteran Seth Moulton. But in the final days before the Massachusetts primary, automated polls showed the congressman with minuscule, single-digit leads.  

It’s clear Tierney also feels the heat on the ground.  

“The fact Barney Frank and Joe Kennedy are campaigning for Tierney, and a brutal ad he ran during the Patriots game at halftime against Moulton tells you he is in trouble,” Mary Anne Marsh, a Massachusetts Democratic operative, said of the 6th District lawmaker. “It is fair to say it is a jump ball that will be decided by ground game Tuesday.”  

Once again, Tierney’s political future relies on his vaunted get-out-the-vote operation in the district north of Boston. The congressman’s turnout operation saved his skin in 2012, when he won re-election by a slim margin; underperforming President Barack Obama by 10 points in his district that year.  

Until now, his political problems were mostly a product of a family ethics scandal. Tierney was accused of improperly reporting to the IRS income his wife received from her brother, who ran an illegal gambling operation. Last September, the House Ethics Committee dropped its investigation into the congressman.  

But the incident remains well known throughout the district. The Boston Globe  mentioned Tierney’s past ethical issues when endorsing Moulton, while the Boston Herald  cited the primary challenger’s “character” in its endorsement of him.  

“The Globe has seized on John Tierney … probably because John’s wife got herself in trouble by trying to take care of her two bum brothers,” said former Rep. Barney Frank, D-Mass. “The Globe, which is generally supportive of Democrats, has been crusading against him not just in editorials but in very unfair coverage. … So that’s what’s made this a close race.”  

Since Sept. 5, three separate polls showed Tierney with a 1-point to 3-point lead over Moulton, all within the margins of error. Two of the surveys, from Public Policy Polling and the student-run Emerson College Polling Society , used an automated system to ask questions. A third firm, Remington Research Group , did live interviews with cellphone users, but the company doesn’t have much of a track record in the Bay State.  

A prolific fundraiser for his entire campaign, Moulton has spent about $1 million blasting Tierney for being ineffective in Congress. In one of his most stinging indictments, Moulton’s campaign charged that Tierney has “only passed one bill” and was “fined $83,000 for illegal campaign activities.”  

“Would you re-elect this congressman?” Moulton’s spot asks.  

Tierney responded with his own attack on Moulton in the final week of the primary.  

“Who’s behind Seth Moulton?” asks an announcer in the spot . “Moulton took money from a special interest group that only funds Republicans. NRA-backed Republicans who voted to outlaw abortions, tea partyers who say they’ll end the Medicare guarantee. Seth Moulton’s special-interest friends gave nearly half a million to 67 Republicans, but donated to just one politician who wants you to believe he’s a Democrat. Huh, what do they know that Seth Moulton won’t tell us?”  

The nine-term congressman could be helped by a muddled primary field that provides frustrated Democrats with more than one Tierney alternative. In addition to Moulton and Tierney, attorney Marisa DeFranco — a liberal who lost to now-Sen. Elizabeth Warren in the 2012 primary — and two other little-known challengers are also running in the Democratic primary.  

“We feel very good actually; we feel very good,” said Dan Rubin, a spokesman for Tierney’s campaign. “We have an incredibly strong campaign, and we have a very solid and well-tested field operation that has been working for months.”  

The winner of the primary will face a top GOP recruit , former state Sen. Richard Tisei. In 2012, Tierney defeated Tisei by fewer than 5,000 votes.  

Privately, some Democrats acknowledge Moulton would be a more formidable general election candidate than Tierney.  

“If Tierney prevails, he goes in [as] not the strongest incumbent,” Marsh said. “You’d still have to say he’s favored to win slightly, and that the challenge for him is the midterm drop off in turnout here.”  

Republican operatives argue party infighting will leave local Democrats fractured. The Democratic nominee will likely start the sprint to Nov. 4 at a cash disadvantage.  

Tierney reported $1.3 million in cash on hand in his last required fundraising report on Aug. 20. But he’s been on the airwaves heavily since that report in the pricey Boston media market, so his campaign bank account has probably shrunk significantly since then.  

Also last month, Tisei reported $754,000 in the bank, and Moulton had about $481,000 in cash on hand.  

No matter the nominee, Democrats argue they are prepared to fight for the district.  

Their playbook  is simple: Portray Tisei as a Republican whose first vote in Congress will be for John A. Boehner for speaker.  

The Democratic Congressional Campaign Committee has reserved  millions  in television time in the Boston market, which covers the district. House Majority PAC , a Democratic super PAC, is also planning to spend about $1.88 million in the Boston media market.  

The National Republican Congressional Committee has also reserved millions in the city, and cash flush Republican groups such as the U.S. Chamber of Commerce and Crossroads GPS could also spend big.  

Massachusetts’ 6th District is rated a Tilts Democratic contest by the Rothenberg Political Report/Roll Call.  

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