(This is the guy who developed City Year, for those who don't know him; I worked for him in the primaries after Kennedy died.)
From his email:
Six months ago I had the honor of beginning a dialogue with the people of Massachusetts and it has been a tremendous experience and an incredible privilege. The citizens of our Commonwealth have been amazing hosts and inspiring thought-partners throughout this campaign.
I’m blessed to have so many wonderful friends and such a loving family – my wife Vanessa, daughter Mirabelle and son Reece. And I’m deeply grateful to my incredible campaign team and volunteers. Truly, as scripture says, my cup runneth over.
...
Elizabeth Warren has struck a chord with citizens across our state and across our country, at all levels of the political process, and I congratulate her doing that so quickly.
I’ve said throughout this campaign that Scott Brown does not deserve to be re-elected because he has failed to lead when our country is crying out for game-changing leadership. I don’t want to do anything that could prevent the defeat of Scott Brown in 2012. And so I believe the best way to be of service is to exit the race at this time.