What's behind Scott Brown's decision not to run for John Kerry's U.S. Senate seat?

Today, Brown released a statement saying he was encouraged to run, that his competitive instincts were leaning him towards another race -- but he won't be a candidate.

On election night, when Scott Brown hinted from the stage of the Park Plaza that he’d be back, analysts and supporters interpreted that as a sure sign he would run again.  

Though Scott Brown said, "There are no obstacles you can't overcome, an defeat is only temporary," many supporters heard him channel Arnold Schwarzenegger -- "I’ll be back."

"He definitely left the door open, and he said defeat is only temporary," said Adriana Cohen, a senior member of Scott Brown’s finance committee and a prolific fundraiser. "A statement, that, to a lot of people, meant the defeat against Elizabeth Warren is only temporary, that he would run again. It definitely gave an indication."

"I'm very surprised he's not going to run again," Cohen said. "On one hand I understand it, because it would mean Scott Brown would have to run in a campaign five times in five years ... that would be hard on any human being."

If Brown were to run it looked like he had a good chance to win.  One poll showed him slightly ahead of  Dem Congressman Ed Markey; another poll gave Brown a  22 point lead over the Democrat. So what's behind Brown's decision not to run?  Some say exhaustion. Former governor Mike Dukakis says all this campaigning  takes a toll. But it could also be strategic. If Brown were to run and lose he would have lost two Senate races in less than eight months. Now he could run for either governor or senate in 2014.  

The party will look for someone else to take on the winner of the Democratic primary in the Spring.   Massachusetts new Republican party chair, Kirsten Hughes, a former Brown deputy finance director says the GOP remains optimistic that it can win an upcoming special. Among the Republican potentials Kerry Healey , the former LT Governor. Former state Senator Richards Tisei -- who says he's evaluating, and former governor  William Weld

Brown fundraiser Adriana Cohen, is not optimistic about the party’s prospects.

"John Walsh, the head of the state Democrat party, he's going to fight hard to retain the seat," she said. "He's stated publicly, 'We didn't just spend $42 million to get Elizabeth Warren in the United States Senate only to turn around and have a republican come in and negate her vote. They're not asleep at the switch."

So what impact will Brown's decision have on the Democratic primary? Based on the polls today's news would seem to be a clear pathway for the Democrats to retain Kerry's senate seat. Lynch is not jumping the gun.   

"I'm sure the Republicans will come up with a credible candidate," Lynch said. "This is a United States Senate race after all."

Browns' decision is also having an impact on national politics.  The Massachusetts race holds special significance for the National Republican Party’s efforts to recapture the US Senate. The GOP would need to win 6 of 8 races to regain the Senate in 2014.  So for now, Brown's political future  as he left it in his final senate floor speech.   

"Defeat is temporary," he said at the time. "Now depending on what happens and where we go, all of us, we may meet again. But I'm looking forward to continuing on with those friendships, continuing working with my staff, and I want to thank you for the opportunity to speak today, and I yield the floor."