"Mike is launching a full-force grassroots effort to collect enough signatures to get on the ballot," said campaign aide Holly Robichaud.

If Sullivan collects the 10,000 signatures needed to get on the ballot by Feb. 27, he will likely face state Rep. Dan Winslow, local businessman Gabriel Gomez and local politician Jon Fetherston in the primary.

Sullivan decided Thursday morning that he was interested in running, and was encouraged, Robichaud said, by the level of support he saw during this past weekend, when volunteers were collecting signatures for him despite a heavy blizzard bearing down on Massachusetts.

"We're going to do this for the next 10 days. It's all volunteer activities, we're not raising a dime. We feel that we have a good enough grassroots organization that we can do it that way," said Robichaud.

Candidates will likely have to collect at least 15,000-16,000 signatures, as thousands are thrown out in the final count, and Sullivan is unlikely to be able to hire a company to help him collect names the way both Gomez and Winslow have.

But Robichaud said Sullivan wasn't concerned.

"We kind of want to show people that we don't need to buy the signatures," she said.

Sullivan served as former President George W. Bush's ATF director during his second term, and also served as Massachusetts U.S. Attorney from 2001-2009.

Winslow and Fetherston also have announced they're pursuing the GOP nomination for Kerry's former seat.

Earlier Thursday, Massachusetts GOP Committeeman Ron Kaufman told The Hill that Sullivan was in the "testing the waters stage," but leaning towards a run.

Democrats will see Reps. Stephen Lynch and Edward MarkeyEdward (Ed) John MarkeyDemocrats search for 51st net neutrality vote Overnight Regulation: Dems claim 50 votes in Senate to block net neutrality repeal | Consumer bureau takes first step to revising payday lending rule | Trump wants to loosen rules on bank loans | Pentagon, FDA to speed up military drug approvals Dems say they have 50 votes in Senate to overrule net neutrality repeal MORE battle it out for the party's nomination in the April 30 primary.

—This post was updated at 5:11 p.m.