Last week, the U.S. Supreme Court heard oral arguments in McCutcheon v. Federal Election Commission, a case many — including my GovBeat colleague Niraj — have dubbed the next Citizens United. McCutcheon challenges the government-set aggregate limits on how much an individual can contribute to federal candidates. It’s the latest salvo in a coordinated drive by conservative lawyers to undermine campaign finance reforms.

And those conservative lawyers aren’t waiting for McCutcheon to be decided before they tee up their next assault — this time on rules against corporations contributing to candidates.

Last week, Indiana attorney Jim Bopp Jr., on behalf of the Iowa Right to Life Committee, asked the U.S. Supreme Court to review Iowa’s ban on political contributions by corporations. Bopp says Iowa’s rules, which allow labor unions to give but prohibit corporations from donating to candidates, violates the Fourteenth Amendment’s equal protection guarantee, along with the right to free speech.

“There’s a really important fairness issue here,” Bopp said in an interview. “Targeting corporations and permitting unions betrays a real partisan agenda, and if we’re going to have these kinds of laws, they’re going to have to, in my view, treat corporations and unions similarly.”

This isn’t Bopp’s first attempt to overturn a ban on corporate contributions. In 2003, Bopp argued before the Supreme Court that North Carolina Right to Life’s right to free speech was infringed by the Federal Election Campaign Act. The Court ruled against Bopp, in a 7-2 opinion authored by Justice David Souter which held that prohibiting direct contributions from nonprofit corporations was consistent with the First Amendment.

(Bopp was the original attorney arguing on behalf of Citizens United, though former Solicitor General Ted Olson and attorney Floyd Abrams argued the case before the Supreme Court.)

Bopp said he thinks the focus on the equal protection element of the case will make it more attractive to the Supreme Court. The court’s decision in Citizens United, Bopp said in a statement, means “there is no justification for banning corporate political contributions because corporations may not be treated differently from other entities. And this is doubly true where Iowa allows labor union contributions, but not corporate contributions.”

The fight to overturn bans on corporate contributions is an uphill battle. Bopp has already lost the argument in U.S. District Court and the 8th Circuit Court of Appeals. But the Colorado Supreme Court ruled in 2010 that the equal protection clause barred the state from permitting union contributions but not corporate contributions. The split decisions, Bopp said, creates a need for the Supreme Court to weigh in.

The case applies to state bans on corporate spending, not the federal ban. Most states prohibit corporations from directly contributing to candidates. But if the Supreme Court sides with Bopp, those state limits, and perhaps even federal limits, could be next to fall.

The odds of Iowa’s ban on corporate contributions are long. But with limits on outside spending gone, and limits on aggregate contributions under assault, it is the logical next step in the decade-long conservative attempt to loosen campaign finance restrictions.