This is somewhat surprising given all the signals Brown had been sending over the last few weeks. But his reasoning appears sound.

Boston Herald:

Scott Brown will not run for the U.S. Senate seat vacated by John Kerry, the former senator told the Herald first today.

"U r the first to know I am not running," Brown texted the Herald.

Brown's decision follows yesterday's announcement by fellow Republican Charlie Baker that he won't run for the seat either. Baker did urge former Gov. William Weld, who just moved back to Massachusetts, to run if Brown doesn't.

[...]

Brown said in a statement this afternoon that being in the Senate was "the greatest privilege" of his life - after the marriage to his wife and the birth of his two daughters - but he's not ready for a third Senate campaign "in less than four years."

"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," Brown wrote.

"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," he added.

"That is why I am announcing today that I will not be a candidate for the United States Senate in the upcoming special election," his statement ends.

If Brown had won in the special election on June 25, he would have had to immediately gear up for another campaign in 2014, making it 4 statewide races in less than 5 years. Not too many politicians could have managed the fund raising necessary in an expensive media state like Massachusetts as well as extend oneself physically and be successful.

Weld is the only credible GOP candidate left, and he may end up being a reluctant participant if the national party can persuade him to run. The former governor appears to be content in private industry, but appeals to his loyalty may overcome his hesitancy.

Otherwise, the Republicans are SOL.