PORTLAND, Maine (AP) — Maine residents this week overwhelmingly approved a referendum to limit donations to political action committees that spend independently in candidate elections, setting the stage for a legal showdown over caps on individual contributions to so-called super PACs that spend freely in elections.

In the nation's only campaign finance reform initiative on the ballot on Election Day, residents voted to cap individual donations to super PACs at $5,000. Supporters fully expect a lawsuit that they hope will bring clarity to PAC donations after the U.S. Supreme Court opened the door to unlimited spending by super PACs.

The measure was carefully crafted to survive legal challenges as states try to find a way to regulate campaign spending after the Supreme Court's 2010 Citizens United decision, said state Sen. Rick Bennett, a supporter of the proposal.

"We've become a place where our democracy is being bought and sold by the richest people in our country," said Bennett, a Republican from Oxford. "People of all political stripes support this measure. The only people who really oppose this are the monied interests who abuse the system."

The Supreme Court opened the floodgates for big business and labor unions to spend freely on elections in the Citizens United decision, and a Court of Appeals decision three months later lifted limits on individual spending.

The Maine initiative doesn't attempt to limit independent spending on behalf of candidates. It focuses instead on limits on individual donations to super PACS, an area the Supreme Court has not ruled on, observers say.

Cara McCormick, leader of Citizens to End Super PACs in Maine, said the goal is to reduce the outsized influence that super PACs currently enjoy through so-called "dark money" spending.

Political nonprofit groups are not required to disclose their donors and do not have to reveal much about how they spend the donations they receive. A super PAC may raise and spend unlimited amounts of money to campaign independently for candidates for federal office. Its activities must be reported to the Federal Elections Commission but is not otherwise regulated if not coordinated with the candidate or campaign.

"We have the right to stand up to the big money in politics in Maine. We're asserting our right to limit the amount of money that someone can give to a super PAC to eliminate not only corruption in our politics but the appearance of corruption in our politics. We think that's something worth fighting for," McCormick said.

In Maine, the limit would only apply to PACs spending money on behalf of candidates, not ballot committees involved in referendums. Maine law currently limits contributions to candidates, not PACs. For general elections, individuals can contribute a maximum of $1,950 to a gubernatorial candidate and $475 to a legislative candidate.

Harvard Law School professor Lawrence Lessig, a longtime advocate for campaign finance reform, and his Equal Citizens nonprofit backed the Maine referendum. A similar citizen initiative effort in Massachusetts was blocked by the attorney general on constitutional concerns.

The U.S. Supreme Court has not ruled on the issue of individual contributions to PACs, and long-established case law supports the notion that states can limit individual contributions to PACs despite a decision to the contrary by the Court of Appeals for the District of Columbia, Lessig said.

The concern with unlimited individual contributions is the risk of a quid pro quo even when super PACs are spending independently and not coordinating directly with a candidate, Lessig said.

The matter will ultimately have to be decided by the Supreme Court.

"I'm very optimistic that the U.S. Supreme Court will apply existing jurisprudence that states are free to limit contributions," he said. "The issue that this case would raise is not asking the Supreme Court to change its jurisprudence, not asking them to overturn Citizens United."

The Maine law goes into effect this winter, if there is no legal challenge, after an emphatic vote. Nearly 75% of voters supported the citizen initiative.