AUGUSTA – Two conservative groups said Friday that they have filed a federal lawsuit challenging key portions of Maine’s public financing system for political candidates.

The lawsuit challenges a provision in the law that gives publicly funded candidates — who are limited in how much they may raise — amounts of money matching what their privately financed opponents raise.

It also challenges the contribution limit of $750 per donor for privately funded candidates for governor.

The Maine Heritage Policy Center and the Indiana-based James Madison Center for Free Speech filed the lawsuit in U.S. District Court in Portland on Thursday. The lawsuit follows similar court challenges in other states, including Arizona, Connecticut and Florida.

Under Maine’s Clean Elections law, candidates for Legislature and governor who raise set amounts through small donations from numerous contributors and refuse to accept large contributions qualify for public campaign funding.

Josiah Neeley, attorney for the plaintiffs, said the matching-fund provision “has a chilling effect on campaign free speech” when a privately funded candidate “knows that every time he puts more money in his campaign war chest, he’s putting more money in his (publicly funded) opponent’s pocket.”

The $750 limit likewise infringes on First Amendment free speech rights, Neeley said. He said the lawsuit doesn’t challenge the legality of a system that provides public funds to candidates.

“You can have a public financing scheme with no matching funds,” Neeley said.

The law’s opponents are seeking to shut off the matching funds in the current election cycle, in which one of the five candidates for governor — Democrat Libby Mitchell — and numerous legislative candidates from both major parties have qualified for public funding.

Eighty-one percent of the current Legislature’s members were publicly financed, according to the state’s campaign watchdog agency.

Efforts to get comment from the Commission on Governmental Ethics and Election Practices, which administers the law, were unsuccessful because state government was closed Friday for a furlough day.

Alison Smith of Maine Citizens for Clean Elections said the lawsuit is without merit and “only the latest attempt in a national strategy” to overturn public campaign finance laws.

Smith cited candidates in Maine’s recent primary races for governor who continued to collect and spend more money for their campaigns to get their messages out, even though their opponents were being matched by the state.

The same has been true in previous elections, she said.

“We see that privately funded candidates who intend to spend a lot of money did so even though they had a Clean Elections opponent,” Smith said.