Maine’s two largest Indian tribes announced Tuesday they have purchased an option on 300 acres in Sanford as a potential site for their proposed $650 million casino.

“It’s exciting to start looking and imagining what it can be,” Erin Lehane, spokeswoman for the Penobscot Nation and the Passamaquoddy Tribe, said of the land deal, the financial specifics of which were not made available Tuesday.

The disclosure came after months of speculation about exactly where the tribes would try to build the massive casino resort, which still must gain voter approval at a statewide referendum in November.

Although the tribes have reserved the right to buy the land near the Sanford airport until after the election, Lehane said they are still considering other locations.

The debate over a casino in southern Maine has been a staple on the pages of the state’s major newspapers since the tribes first floated the idea last year. Nowhere has the debate been more intense than in Sanford, where residents voted in a nonbinding referendum last November to host a casino.

If approved statewide, the project also must gain local zoning and land use approvals.

Sanford officials were notified Tuesday of the tribes’ option to buy the undeveloped land on the southern end of town off Route 109. The land now is owned by Cambridge, Mass.-based construction giant Modern Continental.

The announcement came as a surprise to casino opponents, who warned the tribes against “putting the cart before the horse” by signing the land option before the election.

“I hope they’ve got a way to get out of it,” said Casinos No! spokesman Dennis Bailey, predicting the measure’s defeat in November.

Although the parcel in question is in an industrial area, there are a few homes nearby, said Lehane. She quickly added that if the parcel did become home to the project, developers would seek to place a golf course between neighbors and the casino.

But Bailey said nothing could save the southern Maine town from the increased traffic and other social costs that come with a casino.

“Life as they know it will come to an end,” he said.