Republican Gabriel E. Gomez, fresh off his upset victory, kicked off the special election campaign Wednesday by labeling his Democratic opponent, Edward J. Markey, a “poster boy” for term limits and by refusing to sign a pledge to limit special interest money in the race.
Markey, a 36-year veteran of Congress, quickly hit back at Gomez for saying he personally opposes abortion and would vote against a ban on assault weapons. He warned that Gomez’s rejection of the so-called People’s Pledge to limit outside money would unleash a flood of negative ads in their race.
The vigorous back-and-forth reflected the sudden energy in the Massachusetts Senate race, a day after Gomez, a wealthy investor and former US Navy SEAL with almost no electoral experience, scored a come-from-behind victory against two opponents with much longer roots in the state Republican Party.
As the candidates sprint toward the special election on June 25, the race could become a nationally watched contest with heavy involvement by special interests.
The level of spending from outside groups, however, will be determined largely by polling, some of which has already begun. Strategists for political committees supporting both parties said they want to see whether Gomez will be a viable candidate before committing money to either his campaign or to Markey’s.
One left-leaning super PAC, Next Generation, spent $900,000 in the Democratic primary on behalf of Markey. The group will now conduct its own poll and wants to probe Gomez’s position on global warming before deciding whether to spend money in the special election campaign.
“It depends on the dynamics of the race,” said Chris Lehane, an adviser to Next Generation, which is controlled by Thomas Steyer, a California hedge-fund billionaire. “Obviously, the resources are available, and the commitment is there.”
In the next few days, Gomez will make the case to national Republican backers that he is worth their attention and their money. Ron Kaufman, a member of the national Republican committee from Massachusetts, said he would be setting up Washington meetings for Gomez with big contributors shortly.
Kaufman said that some Republican fund-raisers were waiting to see what happened in the primaries before making financial commitments in the special election.
“People have to get to know Gabe,” he said. “They just don’t know him yet.”
Some Republicans believe Gomez will draw national support, because he is a Latino in a party trying to broaden its appeal, because he is a fresh face challenging a veteran Democrat in deep-blue Massachusetts, and because the contest is currently the only Senate race in the country.
“It’s the only thing on the map,” said Alex Castellanos, a Washington-based Republican strategist who donated $1,000 to Gomez in the primary. “It’s new versus old. It’s a state that has been at the center of the political debate, whether it’s Mitt Romney or Scott Brown. And if Republicans can compete in Massachusetts, Democrats will cower in fear everywhere.”
If they choose to get involved, super PACs will be free to launch ads in the race. Gomez said Wednesday that he would not sign the People’s Pledge to block special interest spending. Brown and Elizabeth Warren pioneered the pledge in their Senate race last year.
Gomez said that he considered the pledge meaningless because Markey has accepted $3.2 million from PACs since 1989, according to the Center for Responsive Politics, which tracks campaign spending.
“It’s the height of hypocrisy,’’ Gomez said of Markey’s proposal, accusing the representative of taking outside money from groups he regulates.
Markey said he would make a major issue of Gomez’s rejection of the pact, which says that if a group runs an ad on behalf of one of the candidates, that candidate must donate 50 percent of the cost of the ad campaign to a charity of his rival’s choosing.
“I am going to challenge Gabriel Gomez every single day, day after day, to take this pledge,” he said at his only stop of the day, a unity event at the Omni Parker House with his vanquished rival from the Democratic primary, Representative Stephen F. Lynch. “And I am not going to stop until the people of Massachusetts force him to take this pledge.”
But Markey, pressed several times by a reporter, also indicated that he would not unilaterally abide by the pledge if Gomez refuses to sign it.
“I believe that in order to do this correctly, you need two people to take the People’s Pledge,” he said.
The League of Conservation Voters is among the groups that could help Markey. During the primary campaign, the league paid for door-to-door canvassers for Markey. The group spent nearly $2 million on television ads attacking Brown before he signed the pact with Warren.
“We’re not going to advertise our strategy,” league spokesman Jeff Gohringer said Wednesday. “We’re certainly committed to make sure Ed Markey wins this race.”
Eric Fehrnstrom, Romney’s former strategist who advised a pro-Gomez super PAC in the primary, did not respond to a call and e-mail.
Americans for Prosperity, a group founded by conservative brothers David and Charles Koch, has generally stayed out of special election races in other parts of the country this year.
“It’s something that’s certainly on our radar, but at this point we haven’t made a commitment one way or another,” said Levi Russell, the group’s director of public affairs.
Despite Markey’s unity event with Lynch Wednesday, he may face difficulties healing the wounds with trade unions that backed Lynch, a former labor leader, during the primary.
The president of the national Building and Construction Trades Department of the AFL-CIO will seek a meeting with Gomez because the union has strong reservations about Markey, said a senior official who requested anonymity because the union has yet to make an endorsement. The union backed Lynch in the primary.
The official said Markey’s vocal opposition to nuclear power and the oil, gas and utility industries puts him at odds with union members who depend on those industries for employment.
Francis X. Callahan Jr., who heads the Building Trades Council at the state level in Massachusetts, declined to discuss the union’s deliberations, saying, “we still haven’t made any commitments yet.”
At the event Wednesday, Markey, who supports legalized abortion and an assault weapons ban, made clear that he considers Gomez’s stances on those issues to be major vulnerabilities.
Gomez said that while he opposes an assault weapons ban, he would support closing the so-called gunshow loophole, so that most gun buyers face a background check.
He also said that while he is personally “prolife,” he considers Roe v. Wade “settled law” and has no intention of trying to change it.
Evan Allen and Joshua Miller of the Globe staff contributed to this report. Michael Levenson can be reached at mlevenson@