NEWTON, Massachusetts — Brian Phillips was wearing flip-flop sandals Sunday at the nondescript office near the corner of Elliot and Chestnut streets that is headquarters for one of this fall’s most remarkable congressional campaigns.

Phillips is campaign manager for Sean Bielat, the young Marine officer who is challenging Barney Frank in one of the most solidly Democratic districts in the country. During his 29-year career in Congress, Frank has never faced such a formidable opponent and the surge of support for Bielat’s campaign has clearly rattled the incumbent. Ever since Scott Brown’s triumph in January, the GOP in this state has been fired up (as Jim Antle noted last week in his article “Red Bay State”) and there is a sense among 4th District Republicans that the Democrats have been caught flat-footed.

“They’ve got no operation over there,” Phillips said of the Frank campaign. As Bielat told reporters at a Sept. 26 rally, “They have one campaign office. It’s always empty. We have three. They’re always full.”

Bielat’s Newton office wasn’t full Sunday morning, but it was open and in full swing, with at least three volunteers calling voters in a telephone canvassing operation that has already reached some 30,000 likely voters and, at a pace of more than 2,000 new contacts daily, could approach the 50,000 mark within a week.

Democrats must contend not only against the most high-intensity GOP campaign organization this district has seen in years, but with a Republican candidate of impeccable qualifications. Bielat is a Marine Corps Reserve officer who got his undergraduate degree at Georgetown, added a master’s degree in public administration from Harvard’s Kennedy School of Government and an MBA from the University of Pennsylvania’s Wharton School of Business. He worked as a program manager for a company that built robots used to destroy roadside bombs in Iraq and Afghanistan. And he talks politics and policy with tremendous fluency.

“We won the primary Sept. 14 and a couple days later, Barney announced he was bringing in Bill Clinton to town [for a fundraiser], which was an eye-opener for a lot of people,” Bielat said in a brief phone interview Sunday. “Two or three days after that, we announced our poll which showed that we were 10 points down with Barney below 50 percent.… Even better, among independents, which make up half the district, we had 51 percent and he had 34 percent.”

Despite its reputation as a bastion of blue-state liberalism, the 4th District was carried in January by Scott Brown, who fared especially well in the southern end of the district from Taunton down toward New Bedford. This part of Massachusetts has been hard hit by the recession — unemployment is at 14 percent in some towns — and Bielat is hammering the incumbent hard on his record as the ranking Democrat on the House Financial Services Committee.

“There is no individual more responsible for what happened with the real estate collapse than Barney Frank,” says Bielat.

In 2003, when the Bush administration and congressional Republicans warned of trouble with the federally sponsored mortgage agencies Fannie Mae and Freddie Mac, Frank denied that the agencies posed any real risk. Frank said the agencies were “fundamentally sound” and accused critics of exaggerating the problems.

“There are a lot of people to blame, a lot of responsibility to go around, but there is no individual who pushed harder for extending home loans to people who couldn’t afford them, and that created the subprime mortgage market, which led to the housing collapse, which led to the financial collapse,” Bielat says. “So [Frank is] at the center of it all.… He pushed hard and it took a lot of pushing to extend these loans to people who couldn’t afford them, and it cost [taxpayers] hundreds of billions of dollars.”

Frank’s vulnerability on his role in the mortgage meltdown has added to the perception that Bielat has a real chance to defeat one of the most powerful (and most outspokenly liberal) Democrats in Congress. That in turn has driven impressive fund-raising to the GOP challenger’s campaign, which raised about $400,000 in September, mostly from small donors making online contributions via RetireBarney.com. For the first time since anyone here can remember, Frank has agreed to debate an opponent, starting this morning on Boston station WRKO, another tonight on New England Cable News, and a third on Tuesday on WFXT-TV.

Like Scott Brown’s January special election, the Bielat campaign has significance far beyond Massachusetts. In a crucial mid-term year, if a Republican can be competitive here, there are very few districts that Democrats can consider “safe,” and the 4th District isn’t even ranked among the 80 most competitive districts this fall.

One volunteer in Bielat’s Newton office, who previously volunteered on the successful GOP Senate campaign, said Sunday she’s beginning to get a familiar feeling.

“This is going national,” the woman named Terri said Sunday. “I smell Scott Brown.”