WASHINGTON — In the ornate meeting rooms of the Senate Committee on Armed Services, the New England senators from neighboring states sat across from each other at dozens of hearings and closed-door budget sessions.
But over three years serving together in the clubby Senate, Republican Scott Brown of Massachusetts and Democrat Jeanne Shaheen of New Hampshire created no bonds — despite their common regional interests and mutual claims of a willingness to work across the aisle — according to interviews with current and former staffers and advisers.
They never argued, true, but they also almost never agreed. Although their signatures appeared on a few of the same letters, and they cosponsored some minor bills (one congratulating the Boston Bruins for winning the 2011 Stanley Cup), that work was performed at the staff level. The two senators rarely interacted and barely spoke.
The unusually cool relationship across the partisan divide helps explain why the New Hampshire Senate race has quickly turned bitter, now that Brown has moved north and is running against the incumbent Shaheen after Elizabeth Warren foiled his own 2012 reelection bid in Massachusetts.
Senate staffers and advisers for each of them— who requested anonymity because they were not authorized to speak by the campaigns — are sniping about the respective opposing candidate.
The lack of any relationship may help explain why Brown showed little hesitancy about running against Shaheen, his ex-colleague.
Current and former Brown advisers say he never had reason to work with Shaheen. One former adviser described Shaheen as “a nothing burger” who “always voted with her party.”
Shaheen’s advisers, meanwhile, say Brown lacked substance and was unwilling to engage in the quiet work necessary to get things done in the Senate. One former adviser referred to Brown as having “very, very shallow depth of understanding or analysis.”
Brown aides dispute the characterization, pointing to three invitations to the White House for bill-signing ceremonies with a Democratic president.
Neither campaign could identify an instance where Shaheen and Brown ever met one-on-one, or even provide an example of a conversation they had. Both declined interview requests from the Globe to discuss their Senate relationship — or lack thereof.
The most that either camp would say is that they always remained on a “cordial’’ footing. In the stilted language of the Senate, cordial is often code for indifferent.
“Scott Brown had a cordial relationship with Jeanne Shaheen, and worked on a collegial basis with her in the Senate,” said Lizzy Guyton, the communications director for Brown’s campaign.
“Senator Shaheen and Senator Brown had a cordial relationship during his time in the Senate,” said Shripal Shah, a spokesman for Shaheen.
Shaheen’s aides say her lack of a relationship with Brown was an outlier. She has worked closely with Senator Johnny Isakson, a Republican from Georgia, on legislation that would put the federal government on a two-year budget cycle. The two of them have appeared together before the Rotary Club of Atlanta.
She has worked with Senator Pat Toomey, a Republican from Pennsylvania, on ending federal crop subsidies for large farming operations, and with Senator Rob Portman, a Republican from Ohio, on energy-efficiency legislation.
She has also worked closely with Senator Kelly Ayotte, her Republican colleague from New Hampshire, as well as Susan Collins, of Maine, on issues such as the Portsmouth Naval Shipyard and New England fishing.
Brown aides pointed to legislation he worked on with several Democrats, including Kay Hagan, of North Carolina, and Joseph Lieberman, an independent from Connecticut who caucused with the Democrats. He also worked with John Kerry, the former senior senator from Massachusetts, and the two of them would occasionally go on bike rides together.
The lack of any relationship with Shaheen may help explain why Brown showed little hesitancy about switching states and running against his former colleague in the Senate, a body that still — despite intensifying partisanship — prides itself as a sociable and exclusive preserve.
Don Ritchie, the Senate historian, could recall only one other instance where two senators who served together later ran against each other: In 1972, Democrat Wayne Morse ran against incumbent Republican Mark Hatfield in Oregon, after the two senators spent two years serving together in the late 1960s.
The partisan differences between Brown and Shaheen also can be viewed through their voting records. Of 178 high-profile votes during the three years they served together, they agreed 48 percent of the time, according to a Globe review of voting records kept by Project Vote Smart, a nonpartisan organization that tracks federal candidates and office holders.
But the voting records also show Brown’s efforts to find issues to side with the Democrats on — and that is one of the only reasons Brown and Shaheen overlapped at all. During 2011 and 2012, Brown voted with his party 66 percent of the time, while Shaheen voted with her party 96 percent of the time, according to a running tally kept by The Washington Post.
Shaheen was the chief sponsor of four bills that Brown cosponsored, including legislation eliminating some loans for sugar producers, and forming a commission of health care experts to advance diabetes care and prevention. Brown was the chief sponsor of only one that Shaheen cosponsored: “A resolution congratulating the Boston Bruins for winning the 2011 Stanley Cup Championship.”
They also were among a group of New England senators who were united against military base closures.
The most significant instance of shared effort ironically became one of Shaheen’s biggest legislative accomplishments.
Shaheen for nearly a year had been pushing for legislation that would cover the costs of an abortion for servicewomen who were the victims of rape or incest. Then, in mid-2012, she got a request she never sought: Brown wanted to join with her.
Aides for Brown, whose reelection at the time was threatened by the Republican Party’s perceived insensitivity on women’s issues, suddenly reached out, eager to sign onto what had become known as the Shaheen Amendment.
Brown had earlier supported legislation that would require the military to do more to report and crack down on sexual assaults. But he had not supported Shaheen’s legislation, which she initially filed in November 2011. Shaheen also never asked him for support.
That changed in May 2012, when the Senate Armed Services Committee began debating a defense reauthorizing funding bill that included Shaheen’s amendment. According to a Shaheen aide, Brown’s aide approached them and said he wanted to cosponsor the legislation.
By that point it was too late. No matter, Brown’s aide said. He would just back it publicly.
“I rise to speak about something very serious, which is the issue of sexual assault in the military, and in support of the Shaheen amendment,” Brown said a few weeks later on the Senate floor. He wrote a letter to House GOP leadership, which had been resisting, and urged them to support the amendment, too.
“It is a simple issue,” Brown said. “Those who are serving in harm’s way who are victims of such horrific crimes should be afforded the same rights as citizens they protect and who rely on federal funding for their health care.”
Brown said that he had cosponsored the amendment, but records show that he was not listed as a cosponsor. He did vote with two other Republicans — Collins, of Maine, and John McCain, of Arizona — to support the amendment in committee in May 2012.
The president of Planned Parenthood, Cecile Richards, sent Brown a letter thanking him for his support.
After the bill passed the Senate during the 2012 lame duck session, Shaheen sent out a press release saying that she and Brown “led a bipartisan group of members from both chambers of Congress in support of the Shaheen Amendment.” Shaheen’s kind words came too late to have any impact on Brown’s election; he had already lost.