Former U.S. Attorney Michael J. Sullivan, a Republican, filed federal paperwork Monday to run in the Senate special election in Massachusetts, becoming the third GOP candidate to officially enter the race.

“Yes, Washington is broken. Yes, it’s dysfunctional. That much is clear,” Sullivan said in a statement. “What we need now is someone who will stand up and say what’s broken and address how we can start to change it. The voters are looking for substance and real alternatives for reform.”

The Massachusetts Senate race represents a long shot for Republicans, and CQ Roll Call rates it as Likely Democratic.

Sullivan joins a GOP field that already includes state Rep. Dan Winslow and Gabriel Gomez, a private equity investor and former Navy SEAL. All three campaigns said they have collected enough certified signatures to appear on April 30 primary ballot.

The winner of the Republican primary will face off against Rep. Edward J. Markey — who is backed by many national Democrats in Washington, D.C., and influential Democrats in the Bay State — or Rep. Stephen F. Lynch, who has significant organized labor support. Markey is the frontrunner.

The general election will be held June 25.