A new lawsuit against Massachusetts is bringing together two disparate sides of the political spectrum. Sort of.

A 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,” says Jim Manley, senior attorney with the Goldwater Institute.

And this disparity played out during a recent big-ticket election, says Paul Craney from the Mass Fiscal Alliance, which is helping with the lawsuit.

“It was the Boston mayoral election that really put this on the map,” Craney says.

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 agrees. She’s with the liberal group Common Cause, and 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 principals of fairness, even-handedness or the rule of law,” Wilmot says. “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 says it refuses to comment on ongoing litigation.