Here are four reasons why:
1) The perpetual campaign: Let’s say Brown runs for the Senate and wins. His reward will be an immediate return to the campaign trail, as he would have to run for reelection yet again in 2014, on the heels of elections in 2010, 2012, and 2013 — and all in the state that provided the most expensive Senate race in the country last year. Even for the most energetic campaigners, that’s a very demanding pace.
2) Democratic question marks in the governor’s race: It’s far from clear who the Democratic gubernatorial nominee will be and how formidable he or she will be. Two top names have recently taken themselves out of the picture. Lt. Gov. Tim Murray, long viewed as a potential Democratic frontrunner, isn’t running. And Attorney General Martha Coakley, who remains popular in the state, has all but ruled out a run. If there's a weak Democratic field and/or an arduous primary (which seems likely), that only increases Brown's chances.
3) GOP success in governor’s races: Massachusetts is a very Democratic state, but when it comes to electing governors, voters have mostly backed GOP candidates during the past two decades. Before current Gov. Deval Patrick (D), the last Democrat to hold the title was Michael Dukakis, who left office in 1991. (Indeed, this is a trend that spans many blue Northeastern states.)
What’s more, Democrats linked Brown to more conservative members of the Senate GOP Conference in the 2012 campaign, and they will likely try to do it again if he runs this year. If he runs for governor, Democrats will try the same thing, but in that case, he won’t be running for a spot under the leadership of Senate Minority Leader Mitch McConnell (R-Ky.). And history suggests that means something.
4) Senate Democrats are determined not to mess up twice: Senate Democrats are resolved to avoid a repeat of 2010 special election, when Brown upset Coakley in a race that once looked like an impossible climb for the GOP. Coakley’s flawed campaign embarrassed national Democrats, who are determined to have a different outcome this time around. The race will be the highest priority for Democratic donors and strategists from day one.
Senate Democrats’ campaign arm, Kerry, and a host of other Democrats are rallying behind veteran Rep. Ed Markey (D). Whether it will work remains to be seen, as Markey will have to get by Rep. Stephen Lynch (D) before he can think about the general election. But Lynch's dabbling with moderate/conservative politics (anti-Obamacare, anti-abortion rights, pro-Iraq war, etc.) and the fact that party leaders are backing Markey suggests Markey is the clear early favorite.
Brown, quite simply, isn't going to catch Democrats off-guard again.
Of course, inspecting the political terrain is just one way to examine a candidate’s prospects. There are other questions, the most important of which is whether Brown misses the Senate, or even wants to be governor. If he loves the Senate and being in Washington, then that's probably the right call for him.
And we would be remiss if we didn’t also look at the arguments for why Brown should and very well might run for the Senate. The special election campaign is short, with voters heading to the polls June 25. So, the burden will be on the Democratic nominee to quickly build name ID to match Brown. Secondly, there is no presidential race at the top of the ticket to boost Democratic turnout. Third, polls show he’d have a good chance. Fourth, he could still run for governor if he loses the Senate race (having two straight statewide losses on your record is hardly ideal heading into a governor’s race, but Massachusetts Republicans don't exactly have a lot of alternatives).
Those are some compelling reasons, but the governor's race still seems like the better bet. As we wait to see whether Brown wants to return to the Senate or not, it’s worth keeping in mind that there’s also an appealing second option on his radar.