Suffolk District Attorney Daniel F. Conley on Sunday called again for a citywide referendum on a proposed East Boston casino and threatened a lawsuit to stop a proposed Everett casino.

Conley, a candidate for mayor of Boston, said all the city’s voters should have a say on a proposal from Suffolk Downs and Caesars Entertainment to build a $1 billion resort casino in East Boston, and residents in that neighborhood should have veto power on the wider vote.

Under the state’s 2011 casino law, such a two-tiered option does not exist. A spokesman for the candidate said that, if elected, Conley would introduce a home-rule petition to enable the option.

He said if that proposal fails and a plan by Wynn Resorts to build an Everett casino moves forward, he will file suit, because residents in Boston and other affected communities have not voted.

“Like the East Boston proposal, the Wynn proposal was voted . . . on in just one community,” Conley said at a City Hall Plaza press conference, “while residents and businesses from Charlestown to Somerville and beyond will bear its traffic, economic, and social burdens but receive none of the proposed benefits.”

Though neighboring communities do not get to vote on casino proposals across the municipal line, state law requires casino developers to negotiate compensation with surrounding cities and towns. Negotiations that fail to produce deals will be settled in arbitration.

He declined to state his position on the East Boston proposal.

The Suffolk Downs and Everett proposals will compete — alongside a third plan from Crossroads Massachusetts to build a casino on vacant land near Interstate 495 in Milford — for a single resort casino license in the Greater Boston area as designated in the state’s casino law.

Boston officials have until Dec. 31 to reach a mitigation agreement with the proponents and hold a public vote on that agreement. The state’s casino law limits the vote in large cities to the ward in which the casino would be built, unless the mayor and the City Council demand a citywide vote.

Conley said the reasoning for limiting the vote to a single ward was a “constitutionally flawed argument” that disenfranchised the city’s other voters.

Celeste Ribeiro Myers, a spokeswoman for the anticasino group No Eastie Casino, said Conley’s proposal made sense.

“I’ve always advocated for East Boston to have first right of refusal. No one’s going to be affected more than the residents of East Boston,” Ribeiro Myers said. “The other piece of that is if I’m a resident anywhere in the city of Boston, I would prefer the opportunity to weigh in.”

Ribeiro Myers also supported Conley’s plan to intervene if the Everett proposal moves forward. “Whoever the mayor is, his first job is to safeguard his residents and businesses across the city,” she said.

A spokesman for Suffolk Downs declined to comment.

A Globe poll found in March that 44 percent of Boston residents support the casino plan, while 37 percent oppose it.

Conley slammed mayoral candidates who support limiting the vote to East Boston. “So far, there has not been one credible argument put forward by any of the candidates arguing that the impact of the casino would end at the boundary lines of East Boston,” he said.

Six candidates said in a recent Globe survey that they opposed a citywide vote: city councilors Felix G. Arroyo, John R. Connolly, Rob Consalvo, and Michael P. Ross; state Representative Martin J. Walsh; and former state representative Charlotte Golar Richie.

Only four backed allowing all Boston residents to vote: Conley, John F. Barros, Charles Clemons Jr., and Bill Walczak.

Walczak and Clemons also opposed the casino proposal. Six supported the plan: Arroyo, Barros, Consalvo, Golar Richie, Ross, and Walsh.

Conley and Connolly left the casino question blank.

On July 30, Walczak, a former health care executive, became the first candidate to aggressively oppose the proposal.