Mr. Brown, a winner in 2010 and a loser in 2012, may be back as a Republican candidate in 2013.
He won't be running against Ben Affleck. The actor-director on Monday took his name off the list of possible Democratic contenders.
But whoever ends up running, assuming that Kerry is confirmed as secretary of State, the race is sure to be a magnet for media attention and money. It would be one of the few opportunities in 2013 for voters to have a say about who sits in Congress at a time of hot fiscal debates and narrow division of power between parties.
Back in 2010, Brown's upset win foreshadowed the conservative gains later that year that would tip the House of Representatives back into Republican control. This time around, the race could become, in part, an early verdict on the performance of both parties on things like tax rates, budget deficits, and how to keep programs like Medicare and Social Security financially healthy.
Whoever wins would have to turn around and start campaigning all over again pretty quickly, since the seat is up for a scheduled vote in 2014.
How did Massachusetts get into this flurry of Senate activity?
Yes, the answer involves the death of one senator and the appointment of another to a presidential cabinet post. But it's also about decisions made in the State House in Boston about how to handle such vacancies.