Brown, in a reversal of his previous position that gun bans should be left up to the states to establish and implement, said after the Sandy Hook shootings that left 20 children and six adults dead that he would be in favor of a federal ban.

"As a state legislator in Massachusetts I supported an assault-weapons ban, thinking other states would follow suit. But unfortunately, they have not, and innocent people are being killed. As a result, I support a federal assault-weapons ban, perhaps like the legislation we have in Massachusetts," he told the Springfield Republican in December.

Brown's record as a state legislator brought him support from the NRA during his first run for Senate, in 2010. During that special election, in which he defeated Democrat Martha Coakley, the NRA spent $59,000 to help him.

In 2012, the group largely stayed out of the race, as did most other outside groups, due to a pact signed by Brown and then-Democratic candidate Elizabeth WarrenElizabeth WarrenDems to unveil ‘better deal’ messaging campaign Monday Juan Williams: Dems finally focus on message Sanders keeping door open on 2020 MORE to restrain the influence of outside money.

However, data collected by the Center for Responsive Politics indicates the NRA spent about $4,000 on the race, and Brown received $5,000 from PACs affiliated with gun rights.

He also, during his second run for office, received more funds than any other senator from the gun rights lobby.

This time around, however, if he runs to replace Sen. John KerryJohn KerrySenators who have felt McCain's wrath talk of their respect for him Dems see huge field emerging to take on Trump Budowsky: Dems need council of war MORE (D-Mass.), who is expected to be confirmed as secretary of State soon, Brown might not be able to count on that support.

His flip-flop on the issue, however — Brown's campaign said during the summer, following the Aurora theater shootings, that the matter should be left to the states — might be more politically expedient for a Republican running in a deep-blue state.

Brown ran in 2012 arguing primarily that he was an "independent voice" for Massachusetts, but made no secret of the issues on which he agreed with President Obama, even touting his work with the president on legislation. That's because the president is wildly popular in Massachusetts, winning the state with 60 percent of the vote.

And going forward, Democrats will be looking for any opportunity, as they did in 2012, to align Brown with the Republican Party, framing a vote for Brown, who remains popular in the state, as a vote for the decidedly less popular GOP.

Brown worked to hedge against this argument in his about-face on the assault-weapons ban, a position that, while it might lose him the support of the NRA, might not end up costing him much in the final sum.

--This post was updated at 10:30 a.m. to reflect money spent by gun groups.