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 such -so-called free-spending super PACs. in the elections.

In the nation’s only campaign finance reform initiative on the Election Day ballot, residents voted to cap individual donations to super PACs at $5,000. Advocates 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 2010 Citizens United decisionsaid state Sen. Rick Bennett, a proponent of the proposal.

“We’ve become a place where our democracy is bought and sold by the richest people in our country,” said Bennett, an Oxford Republican. “People of all political stripes support this measure. The only people who are really opposed to this are the moneyed interests who abuse the system.”

The Supreme Court opened the door for big business and 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.

Maine’s initiative does not seek 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 huge influence super PACs currently enjoy through so-called “dark money.”

Nonprofit political groups are not required to disclose their donors and do not have to disclose much about how they spend the donations they receive. A super PAC can raise and spend unlimited amounts of money to independently campaign for candidates for federal office. Its activities must be reported to the Federal Election Commission, but are not otherwise regulated unless coordinated with the candidate or campaign.

“We have a right to stand up to the big money in Maine politics. We assert our right to limit the amount of money 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 it’s something worth fighting for,” McCormick said.

In Maine, the limit would apply only to PACs that spend money on behalf of candidates, not to voting 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 campaign finance reform advocate, and his nonprofit Equal Citizens supported the Maine referendum. A similar citizen initiative effort in Massachusetts was blocked by the attorney general on constitutional grounds.

The U.S. Supreme Court has not ruled on the issue of individual contributions to the CAP, and long-standing case law supports the idea that states can limit individual contributions to the CAP, 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 a counterparty risk even when super PACs spend independently and don’t coordinate directly with a candidate, Lessig said.

Ultimately, the matter will have to be decided by the Supreme Court.

“I’m very optimistic that the U.S. Supreme Court will apply existing case law that states are free to limit contributions,” he said. “The issue this case raises is not asking the Supreme Court to change its case law, nor asking them to overturn Citizens United.”

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