Scott Brown, who surprised the political world with his upset victory in the 2010 special election, announced Friday afternoon that he will not enter the special election to replace John F. Kerry.

“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,” Brown said in a statement. “And I know it’s not the only way for me to advance the ideals and causes that matter most to me.”

Brown’s announcement was unusual. Rather than a formal press conference or statement, he initially released the news to the Boston Herald in a text message that said “U are the first to know.” His spokesman later confirmed the news to the Globe in a text that read “Not running.”

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The full statement was released later.

Brown’s decision leaves the Republican Party scrambling to find a viable candidate for the June 25 election. To make the ballot, candidates must gather 10,000 certified signatures in four weeks.

Brown had long been considered the party’s strongest and most likely candidate. The party may now turn to former governor William F. Weld or former lieutenant governor Kerry Healey.

Beyond that, the list of credible candidates is thin. Weld has left open the possibility he would run, but associates say he is unlikely to leave his law and consulting practice to resume a political career.

Healey has not ruled out a run, but has said she was hoping that Brown would be the nominee.

Other possible Republican candidates Gabriel Gomez, a businessman and former Navy SEAL from Cohasset, and State Representative Daniel Winslow, a former district court judge who has served in the Romney administration and has higher political aspirations.

State committeeman Steve Aylward said his fellow activists are upset that Brown bowed out immediately after his chosen chairman was elected to lead the party. Kirsten Hughes only narrowly won the chairmanship Thursday night, after a big push by Brown to get her elected.

“One of the reasons for voting for Kirsten was that Scott Brown was running for Senate and needed her to be there,” said Aylward, who supported Hughes’ opponent. “Now one day afterward, he decides not to run for it? I think I’m speaking for a lot of the grassroots activists who are going to say, ‘this doesn’t pass the smell test.’ ”

Brown, in his statement, thanked voters and reflected on his three-year tenure in the Senate, but gave no hints about his future plans.

“When I was first sent to the Senate in early 2010, it wasn’t exactly welcome news for President Obama or many other Democrats,” he said. “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.

Two Democrats, US Representatives Edward Markey and Stephen Lynch, are vying for their party’s nomination.

Lynch, who is trying to appeal to the same moderate and conservative blue collar voters that Brown captured, released a statement about Brown’s decision before Brown’s even went out.

“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.”

The Globe reported today that Washington Republicans had mounted a “full court press” to persuade Brown to run.

The effort, coordinated by the National Republican Senatorial Committee, came as those familiar with Brown’s thinking were becoming concerned that he would not run and would instead seek a job in the private sector.

The national GOP’s pitch to Brown was that he was at the height of his popularity and had a campaign and fund-raising structure still in place from his unsuccessful race against Elizabeth Warren.

But Brown had told state Republicans and others in his circle that he was exhausted from his campaigns in 2010 and 2012. He has noted to friends that if he won the election, he would have to gear up for a fourth campaign in 2014 to win a full term.

Brown may yet enter the race for governor in 2014. Republicans have generally fared better in races for state office than in races for federal office in Massachusetts.

After losing the November election to Elizabeth Warren, Brown had dropped several strong hints that he would run again for the Senate.