A long-standing provision in Minnesota's campaign finance regulations is getting renewed scrutiny after a U.S. Supreme Court decision that swept aside some federal contribution limits.

For decades, Minnesota's law has said campaigns can raise only 20 percent of their cash from lobbyists, political action committees and donors who contribute large amounts. After candidates hit that limit, they only can accept lesser donation amounts from subsequent contributors.

Opponents see that provision as ripe for challenge, given recent U.S. Supreme Court decisions opening the door to more money in politics. They say Minnesota's "first come, first served" law is an unconstitutional limit of free speech.

"What this law says is the first people to speak have full rights, but subsequent speakers have half rights," said Lee McGrath, executive director of the Institute for Justice Minnesota Chapter. The libertarian Institute for Justice plans to file a lawsuit challenging the constitutionality of the Minnesota law on Wednesday.

As a practical matter, the current law means that this year's crop of gubernatorial candidates can each accept donations worth $730,200 from lobbyists, political funds, PACs or major donors. Before candidates hit that limit, they can accept up to $4,000 in donations from any individual. After candidates hit the $730,200 limit, they can accept only $2,000 contributions per individual and they cannot take any money from political funds, PACs or lobbyists.

Katharine Tinucci, campaign manager for Gov. Mark Dayton, said that Dayton's re-election campaign has either reached or is near its $730,200 limit this year.

For House candidates, the limits are much lower. House candidates can accept up to $1,000 in donations from individuals, lobbyists, political funds and PACs. But once they hit a collective total of $12,500, no individual can give more than $500.

"This lawsuit will ask why it is OK for the first 12 people to make $1,000 contributions but not OK for anyone else to do so," the Institute for Justice said in a statement.

Minnesota is among 10 states that place similar limits on what candidates can accept from certain groups.

Whether the law is constitutional or not, it has leveled the playing field between incumbents and challengers, said Gregg Peppin, a former consultant to legislative campaigns now working with Republican Jeff Johnson's gubernatorial campaign.

"It's a limit on what incumbents could receive from an unlimited spigot," Peppin said. "The incumbents are going to reach that max a lot sooner."

The Minnesota limits are significantly different from the federal limits the U.S. Supreme Court struck down last week. Those now-negated federal limits blocked donors from giving more than $123,200 to federal candidates, party committees or PACs. The court, essentially, ruled that limiting how many contributions a donor could give across all federal campaigns created an unconstitutional obstacle to the exercise of First Amendment rights.

Minnesota does not limit donors' ability to give to as many candidates as they want and places no ceiling on contributions to PACs or parties. Last year alone, one couple — Harold and Eleanor Hamilton — gave more than $46,000 to Republican candidates across Minnesota. In 2013, Alida Messinger gave $640,000 to DFL Party committees.

Although those donors can give freely, if they give late, some candidates may not be able to accept as much money from them as they accepted from others.

That unusual limit on candidates is now being questioned.

"The government cannot put restrictions on speech that treat one speaker differently from another speaker," McGrath said.