“That is why I am announcing today that I will not be a candidate for the United States Senate in the upcoming special election.”
Early polls suggested Brown, who remains personally popular in deep-blue Massachusetts despite his party affiliation, would have started with a lead on either Rep. Ed Markey (D) or Rep. Stephen Lynch (D). One poll even put him ahead of Markey, the Democratic favorite, by 22 points.
But the race was expected to be much closer than that, and even if Brown had won, he would have faced another reelection campaign in 2014 -- his fourth Senate campaign in five years.
"I respect Scott Brown’s decision and know that he did what he thought was best for him and his family," Markey said in a statement. Lynch said it was "perfectly understandable" that Brown "wouldn't want to undertake another campaign."
Republicans are now without a top candidate for the seat held by Kerry, who was confirmed by the Senate to be the next secretary of state earlier this week.
With Brown out of the picture, potential GOP candidates include former state senator Richard Tisei, former governor Bill Weld, former Lt. Gov. Kerry Healey and businessman and former Navy SEAL Gabriel Gomez, whom national Republicans have been recruiting.
Tisei could be an attractive option for the GOP as an outside-the-box candidate. He is openly gay and pro-abortion rights and proved to be a strong fundraiser last year. He is looking at a rematch with Rep. John Tierney (D) in 2012, but also said Friday that he would look at a Senate run.
"Scott's exit from the race was obviously unexpected," Tisei said in a statement. "That said, in the coming days I will be talking with family, friends, and supporters to consider the best role that I can play in helping to bring new, alternative leadership to Washington."
Weld, who recently returned to Boston from New York, told the Boston Globe in December that he didn't think he would run for the seat, but he didn't quite rule it out, either. He ran for Senate before, losing a competitive race with Kerry in 1996. He also ran unsuccessfully for governor of New York in 2006.
Healey served as lieutenant governor under Mitt Romney for one term but lost badly we she attempted to succeed Romney for the state's top job in 2006.
Gomez, 47, is the least known of the three but appears to be the most eager, saying this week that he would even consider challenging Brown in the primary.
One Republican who won't run is 2010 gubernatorial nominee Charlie Baker. He is expected to seek that office again in 2014 instead.
While Republicans will certainly be underdogs in the special election, they were also big underdogs in 2010, when Brown surged to victory as a little-known state senator. But Democrats insist they won't be caught off-guard as they were back then.
The special primary will be April 30; the special general election will be June 25. The winner of the election will have to campaign for reelection in 2014.
Gov. Deval Patrick (D) has appointed his former chief of staff, Mo Cowan, as interim replacement for Kerry until the special election produces a winner.
Brown, who was elected in 2010, lost his seat last November to now-Sen. Elizabeth Warren (D). It's worth keeping an eye on his political future, even though he is not running for Senate. The 2014 governor's race will be an open contest that may interest Brown.
Here's Brown's full statement:
“Representing Massachusetts in the United States Senate was the greatest privilege of my life, an experience that takes second place only to my marriage to Gail and the birth of our daughters. It was a higher honor than I had ever expected, and in the time given to me I always tried to make the most of it.
“When I was first sent to the Senate in early 2010, it wasn’t exactly welcome news for President Obama or many other Democrats. Yet among my best memories from those three years in office are visits to the White House to see the President sign into law bills that I had sponsored. I left office last month on the best of terms with colleagues both Republican and Democrat. I had worked well with so many of them, regardless of party, to serve the public interest just as we are all supposed to. All of this was in keeping with the pledge I made at the beginning to do my own thinking and to speak for the independent spirit of our great state.
“Over these past few weeks I have given serious thought about the possibility of running again, as events have created another vacancy requiring another special election. I have received a lot of encouragement from friends and supporters to become a candidate, and my competitive instincts were leading in the same direction.
“Even so, I was not at all certain that a third Senate campaign in less than four years, and the prospect of returning to a Congress even more partisan than the one I left, was really the best way for me to continue in public service at this time. And I know it’s not the only way for me to advance the ideals and causes that matter most to me.
“That is why I am announcing today that I will not be a candidate for the United States Senate in the upcoming special election.”
Updated at 4:40 p.m.