AUGUSTA, Maine — A key provision of Maine’s public campaign financing program will have to be rewritten following a U.S. Supreme Court ruling on Monday striking down a nearly identical law on the books in Arizona.

In a 5-4 decision, the Supreme Court declared as unconstitutional an Arizona law that gives extra cash to publicly funded candidates who are being outspent by their privately funded rivals.

Writing for the majority, Chief Justice John Roberts said matching funds violate the First Amendment because they “inhibit robust and wide-open political debate without sufficient justification,” The court made clear, however, that public campaign financing laws, in general, are constitutional.

Court observers had anticipated such a ruling, so much so that the Maine Ethics Commission  — which administers Maine’s Clean Elections program — is seeking lawmakers’ approval to study the ruling and recommend fixes in time for next year’s legislative session. That bill faces additional votes in the Legislature this week.

Maine’s matching funds program also had been the subject of a court challenge that was shelved pending the Supreme Court decision on the Arizona case, so it was not surprising that Monday’s ruling generated mixed reactions.

“It is certainly disappointing, but it is not unexpected,” said Alison Smith, co-president of Maine Citizens for Clean Elections, a nonprofit group that advocates in support of public campaign financing.

“The Roberts court has obviously very aggressively acted to reshape campaign finance law,” Smith said. “This is just one in a series of decisions … to roll back campaign finance laws.”

The Maine Heritage Policy Center, which had challenged the Clean Elections program’s matching funds in court, cheered the decision.

“Today’s landmark decision upholding the free speech rights of political candidates and independent groups in Arizona shows just how close we are to finally repealing Maine’s unconstitutional matching funds scheme,” David Crocker, director of MHPC’s Center for Constitutional Government, said in a statement. “These matching fund schemes pick and choose whose free speech rights should be enhanced, and whose should be restricted, and that is in clear violation of the First Amendment rights all citizens are guaranteed.”

Maine’s public campaign financing system is a popular option for candidates from both political parties. In 2010, 77 percent of legislative candidates — including 94 percent of Republican Senate candidates — opted to participate in the Clean Elections program, thereby forgoing private donations.

Under both the Arizona law struck down Monday and in Maine, lawmakers participating in the public campaign financing system are entitled to receive additional money if their privately financed opponents spend more than them. Outside groups spending money to oppose a publicly financed candidate can also trigger matching funds.

In Maine, matching funds consume roughly 25 percent of the Clean Elections Act funding for legislative candidates but a much higher percentage in gubernatorial races. In last year’s race for the Blaine House, for example, the Ethics Commission paid out just shy of $3 million in Clean Elections funding, $1.2 million of which was matching grants, according to figures supplied by commission staff.

Supporters of public financing systems insist they reduce the influence of interest groups and large donors because participating candidates are prohibited from accepting private donations.

But critics contend the matching funds are a violation of the First Amendment because they discourage privately financed candidates or outside groups from spending money. And five Supreme Court justices agreed, ruling that the matching funds program “substantially burdens protected political speech without serving a compelling state interest.”

As lawmakers return to Augusta on Tuesday to wrap up the 2011 legislative session, one bill on their agenda is LD 848, which would direct the Ethics Commission to study the ruling and recommend possible changes in time for the 2012 legislative elections.

The bill has breezed through the Legislature so far. But it could face additional scrutiny if it reaches the desk of Republican Gov. Paul LePage, who has supported ending public campaign financing for gubernatorial candidates.

LD 848 would cost an estimated $3,250 for the Legislature’s Veterans and Legal Affairs Committee to meet out-of-session and recommend a possible bill. LePage has cited financial concerns when he vetoed several bills that sought to study other issues. The governor’s office declined to comment Monday on how he might respond to LD 848.

Jonathan Wayne, executive director of the Ethics Commission, said matching funds are a small but important part of the public campaign financing program because it assures candidates they will be able to respond to heavy spending by an opponent or outside groups.

“This resolve is very important to having a predictable and orderly election in 2012,” Wayne said. “This is a study that will have a very real impact in the short-term so I hope it moves forward.”