If Kennedy passes, Capuano would likely become the frontrunner in the race, and Bay State Republicans would welcome him as the nominee because he is much farther the left than Brown and, consequently, would leave those centrist voters open to Brown. His lackluster primary performance against Coakley didn't draw him many new backers, either.
After Capuano there is Rep. Stephen Lynch (D) -- the most conservative member of the congressional delegation. Lynch would likely only emerge from the Democratic primary if several Democrats to his left also run and split up the liberal vote.
With Massachusetts projected to lose a congressional seat, any of its members could choose to run for the Senate to avoid facing off against another incumbent after reapportionment. The dean of the delegation, Rep. Edward Markey (D), would be at the top of that list, but Democratic Massachusetts sources say it's unlikely he'll run. Same goes for Reps. Barney Frank (D), Niki Tsongas (D) and John Tierney (D). City Year Alan Khazei, who ran in the special election primary, may also run again.
Brown also starts out with a significant fundraising advantage against any challenger. He currently has nearly $6.8 million in his campaign account. Capuano has just $144,000 in his warchest, though Democrats believe he would be able to raise funds quickly by tapping into the national progressive fundraising base.
Brown has also worked to increase his fundraising operation, recently hiring John Cook, who headed Republican Charlie Baker's fundraising team during the gubernatorial race this year. Brown's inner circle believes he will have more money for his re-election campaign than any other senator up for re-election in 2012.
This doesn't mean Brown will skate to re-election. "I mean, come on, it's a Republican in Massachusetts," quipped one Bay State Republican strategist. "I would stipulate that Brown is the underdog," added Fehrnstrom.
And Republicans point to the virtually non-existent GOP infrastructure in Massachusetts as an obstacle Brown will have to overcome.
"A big problem for Brown is the wipe out the Republican Party has suffered here," said Massachusetts Republican strategist Todd Domke. "The party infrastructure hasn't' gotten much stronger."
Further, Brown has gotten some negative press of late, particularly for voting against the repeal of "Don't Ask, Don't Tell" earlier this month.
Brown has also made a few political missteps, most notably backing scandal-plagued State Rep. Jeff Perry (R) in his bid for Congress this year. Brown stood by Perry even as details about Perry involvement in the illegal strip search of a woman when he was a police officer emerged. Perry went on to lose to Democrat Bill Keating.
One potentially significant factor in 2012 is how much Obama's re-election bid helps the Democratic nominee against Brown. Massachusetts gave Obama more money per capita than any other state in 2008, but even Democrats in the Bay State acknowledge that Obama's popularity in the state has suffered.
"It all depends on whether Obama has recaptured some of his luster," Payne said. "If he is popular and he reignites his base, he'll help the Democrat but it's too early to tell."
Brown also has a history of attracting split ticket voters. In 2004, he won a special election for his state Senate seat that Democrats purposely scheduled on the same day as the Massachusetts Democratic presidential primary -- when Bay State Sen. John Kerry (D) was running for the nomination. Brown won anyway. Brown followed that up in 2008 when Republicans were virtually decimated in Massachusetts. Despite Democrats pouring resources into the race to unseat him and Obama winning Massachusetts by more than 26 points, Brown won his state legislative district with 59 percent.