A new lawsuit against Massachusetts is bringing together opposite sides of the political spectrum — sort of.

The Goldwater Institute, a Phoenix, Ariz.-based conservative think tank, is suing Massachusetts over a rule that bans businesses from making any direct political donations but allows unions to contribute up to $15,000 to candidates.

"We’re asking the court to strike down the discrimination in Massachusetts law against businesses and in favor of labor unions," said Goldwater Institute attorney Jim Manley.

This disparity played out during the last Boston mayoral election, says Paul Craney of the Mass Fiscal Alliance, which is helping Goldwater with the lawsuit.

"It was the Boston's mayoral election that really put this on the map," Craney said.

Unions gave more than $600,000 to the eventual winner of that race, Marty Walsh.

This lawsuit says whatever campaign finance limits apply should apply equally to businesses and unions.

Pam Wilmot, executive director of liberal advocacy group Common Cause, agrees. She says the loophole that lets unions get around the law was issued by an outside regulatory agency in 1988.

“I see no way in which that ruling by the campaign finance office, serves the principles of fairness, even-handedness or the rule of law," Wilmot said. "It’s just something that’s indefensible in my opinion."

But the agreement between the left and the right only goes so far. The Goldwater institute doesn’t believe anyone should be barred from making political contributions. Common Cause believes both businesses and unions should be barred.

The legislature has considered removing the loophole but has never actually done it. The Massachusetts Office of Campaign and Political Finance said it refuses to comment on ongoing litigation.