“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," Brown said in a statement. "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."

But with Brown out, Republicans have been scrambling to find a candidate for the June 25 election. State Senate minority leader Richard Tisei, the choice of many in the party after Brown opted not to run, also announced over the weekend that he would not seek the nomination.

“I believe it’s imperative that the Republican party put forward a strong candidate who can help bring much-needed change to Washington,” Tisei said in a statement. “Unfortunately, the timing is simply not right for me to do so — deeply as I feel about the need to strongly compete in this election. It was also my desire to make this decision as quickly as possible so that other potential candidates would be able to consider whether they should run.”

The Boston Herald reported Monday that Tagg Romney, the eldest son of 2012 GOP presidential nominee and former Massachusetts Gov. Mitt Romney, is weighing a run. Romney's former lieutenant governor Kerry Healey is also thought to be a potential candidate for the spot, as is Fox News contributor and psychiatrist Keith Ablow.

Whoever is the eventual nominee, he or she will face an uphill battle in the heavily Democratic state. Reps. Edward MarkeyEdward (Ed) John MarkeyHillicon Valley: Lawmakers target Chinese tech giants | Dems move to save top cyber post | Trump gets a new CIA chief | Ryan delays election security briefing | Twitter CEO meets lawmakers Twitter CEO meets with lawmakers to talk net neutrality, privacy Senate votes to save net neutrality rules MORE (D-Mass.) and Stephen Lynch (D-Mass.) have both signaled their intention to seek the Democratic nomination.