PORTLAND, Maine — A campaign to persuade voters to approve a southern Maine casino reported it has paid over half a million dollars to the firm that has in the past helped persuade British voters to withdraw from the European Union.

Progress for Maine PAC reported Thursday spending more than $1.5 million this year on consultants, canvassers, advertisements, legal fees and mailers, for the pro-casino campaign. It has paid nearly $575,000 through September to Washington, D.C.-based firm Goddard Gunster.

The leading organization that campaigned for the United Kingdom to exit the European Union had hired Goddard Gunster, calling it one of the world’s leading referendum and issue management firms.

State voters are set to consider a Nov. 7 ballot question that will ask them whether they would allow a developer to apply to build a York County casino. Officials behind the campaign have identified an Old Orchard Beach campground and mobile home park as a potential site.

Gambling entrepreneur Shawn Scott and his associates are the only groups allowed to apply for such a license, according to restrictions in the referendum. Scott previously financed a successful campaign to bring a Bangor race track and casino, and slots parlor to the state.

Maine’s ethics commission has been investigating since this summer the source of millions of dollars in contributions behind the casino effort. Horseracing Jobs Fairness, the committee that initially launched the casino effort, reported receiving $4.3 million from several companies linked to Scott and his associates.

An anti-casino effort, a Bad Deal for Maine, reported receiving $26,965 worth of polling services from Kentucky-based Churchill Downs Incorporated, which owns and operates a casino in Oxford, Maine.

The pro- casino effort reported more contributions than a referendum effort to expand Medicaid in Maine, according to state campaign finance reports.