John Richardson’s withdrawal from the gubernatorial race is the latest twist in the 10-year-history of the law that began public financing of state elections.

Maine voters approved the first-in-the-nation public financing system, the Maine Clean Election Act, in a 1996 referendum. Legislative candidates started receiving public money in 2000.

In 2008, 81 percent of legislative candidates participated, according to the Maine Commission on Governmental Ethics and Election Practices.

“It’s opened the door to lots of candidates who would not have run for office,” said Alison Smith, co-chair of Maine Citizens for Clean Elections, which helped put the measure on the ballot.

The three gubernatorial candidates who have qualified for public funds this spring — Democrats Patrick McGowan and Elizabeth Mitchell and Republican Peter Mills — will receive $400,000 each for primary campaigns. They will be eligible for more if the ethics commission determines their opponents and third-party interest groups are outspending them.

If the eventual Republican and Democratic nominees are clean election candidates, they will receive $600,000 to start, with the possibility of $1.2 million more, depending on opponents’ spending. They cannot collect any other contributions.

Four years ago, the ethics commission denied public funding to an independent gubernatorial candidate, John Michael, saying fewer than 2,000 of the contributions Michael submitted were legitimate. As in Richardson’s case, some people reported as contributors said they hadn’t donated to Michael’s campaign.

Clean election rules changed in 2007. Campaign workers now must sign documents acknowledging they have collected money; money orders must be signed by contributors; and candidates and their treasurers must review the process with the ethics commission staff.