Former U.S. Sen. Scott Brown has decided not to run for Secretary of State John Kerry's senate seat, setting off a flurry of reactions from rivals and observers. 

U.S. Rep. Stephen Lynch, who threw his own hat in the ring this week, issued a sympathetic statement.

“I understand Scott Brown’s decision," Lynch said. "He has basically been campaigning non-stop for three years.  It’s perfectly understandable that he wouldn’t want to undertake another campaign.  I wish all the best to Scott and his family.”

U.S. Rep. Dan Winslow (R—Norfolk) issued a statement saying he is honored by the public desire for him to run for Kerry’s seat, but that he is not yet sure if he will campaign.

“In my public service as a State Representative, a Judge, and as Legal Counsel to a Governor during a time of fiscal crisis, I have worked with people of diverse backgrounds and beliefs, all united by the common goal that we can make Massachusetts a better place for ourselves and our families,” Winslow said.

“Massachusetts Republicans, especially middle class Republicans who call Amherst their hometown, seem offer a different perspective from the national GOP brand.  I am honored by so many calls and emails urging me to run for the United States Senate,” he continued. “But I will reflect this weekend about my own family's needs and whether there is room in the national Republican Party for a member who is both fiscally prudent and socially tolerant.”

University of Massachusetts Professor Mo Cunningham agreed with Rep. Lynch, saying you can't underestimate the difficulty of being a Massachusetts Republican.

"I think the national Republican party has re-killed him a second time. They killed him in 2012 and they would kill him in 2013," Cunningham said. "That’s how tough it is to run as a Republican in this state.”

Massachusetts Democratic Party Spokesperson Samantha Hooper said the party is eager to elect a candidate who will support the president.

"Massachusetts Democrats are ready and organized to elect a senator to succeed John Kerry who will stand up for the values of Massachusetts voters and support President Obama's agenda to create jobs and keep our economy moving forward," she said.

Rob Eno, the publisher of Red Mass Group, expressed his disappointment while saying that he understood Brown's decision.

"You know, it's disappointing. I think a lot of people, especially at last night's Republican State Committee meeting, thought that Scott Brown was going to run. He played it really close to the vest, and he's taken a lot of people by surprise. Everyone was ready to jump behind Scott and help him win the Senate seat again, and I think everybody thought he could," Eno said.

"But you can see—it's a grueling thing to run for US Senate and have three races in, what is it, 36 months?  To run three campaigns of that magnitude in 36 or 40 months would weigh and grate on anybody. He's looked at it, and for sanity of his family — it's tough for him and for anybody to put his family through 40 straight months of that," he continued. "I think that played into his decision, I wish him all the best, and we'll see if he has anything else in store in the future. To be frank, nobody seems to know what he's thinking except Scott."

Massachusetts GOP Chairman Kristen Hughes said that the party remains optimistic about the upcoming special election.

“The Massachusetts Republican Party is optimistic about the special election for U.S. Senate because of the strength, character and accomplishments of the many potential candidates who are today considering their political futures. The fact is the Democratic Party will field a mediocre congressman with a highly partisan record who has been part of the Washington gridlock. A Republican Senator from Massachusetts will offer the bipartisan leadership to solve our nation's problems,” Hughes said in a statement.

“Members of our party have an unparalleled level of energy and commitment to the Republican cause. This special election will unite us in the shared purpose of again electing a Republican to the United States Senate. We shocked the world in 2010, and united, we can do it again,” she concluded.

Forensic psychologist and New York Times bestselling author Keith Ablow said that he would consider running if Former Massachusetts Gov. Bill Weld does not enter the race.

"I'm anxious to see what Gov. Weld has in mind," Ablow said. "I'll be discussing it with my family and advisers, given the senator's decision."

Immigration attorney and 2012 senate candidate Marisa DeFranco told WGBH News she is "seriously considering" entering the senate primary, saying Brown's absence "takes out one more obstacle to the race."

Former Gubernatorial Candidate Charlie Baker: "These are intensely personal decisions, and while I think Scott would have been an outstanding candidate, I respect his decision not to run in the special election."

Reaction from Scott Brown himself:

Scott Brown leaving Milford's D'Angelo's:"my statement speaks for itself"when asked why not running #MAsen #MApoli #breaking

"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..." #masen

"..more partisan than the one I left, was really the best way for me to continue in public service at this time." - Brown #mapoli #masen

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