A federal appeals court ruled Tuesday that a state can cap political donors’ campaign contributions only if it proves those limits are preventing corruption or the appearance of corruption.

States can limit contributions in election campaigns if they have a legitimate interest in doing so. But proving that state interest has changed since the U.S. Supreme Court’s 2010 ruling in the Citizens United case that said corporations can spend unlimited amounts in elections, a three-judge panel for the 9th U.S. Circuit Court of Appeals said.

Before the Citizens United decision, states only had to show they aimed to curb the influence of big money on politicians. After Citizens United, states must show more specifically that their laws are stopping an exchange of money for political favors, the opinion written by Judge Carlos Bea said.

“Now, the prevention of quid pro quo corruption, or its appearance, is the only sufficiently important state interest to justify limits on campaign contribution,” Bea wrote.

The ruling, which applies to the nine Western states in the 9th Circuit’s district, was made in a case that challenged Montana’s contribution limits as violating donors’ rights to free speech and association.

The attorney for the plaintiffs in that case, James Bopp, said the ruling is significant and creates a high bar for states to keep their contribution limits.

“The issue is a very new issue. This is the first decision of its kind,” Bopp said.

U.S. District Judge Charles Lovell of Helena used the pre-Citizens United standard in determining there was a state interest for Montana’s campaign-contribution limits. The judge ultimately found the limits were unconstitutional for a different reason: they are too low for candidates to effectively campaign.

The appeals court reversed Lovell and sent the case back the judge with orders to apply the post-Citizens United standard. It said the judge must decide whether the state attorney general’s office has proven its argument that contribution limits further the state’s interest and to identify what that interest is.

The 9th Circuit’s decision means Lovell must carefully measure whether the limits are fair to donors, candidates and the citizens who want fair and equal status in their elections, Montana Commissioner of Political Practices Jonathan Motl said.

“I’m firmly confident that the contribution limits will be sustained,” Motl said.

Montana’s individual contribution limits are $170 for legislative candidates, $320 for attorney general candidates and $650 for gubernatorial candidates. The law also sets total contribution limits from political party committees to candidates that range from $850 per election for a state House candidate to $23,350 for a gubernatorial candidate.

Lovell struck down the contribution limits a few weeks before the 2012 election. The 9th Circuit blocked the ruling just days after the state filed an appeal, but not before the Montana Republican Party used Lovell’s ruling to donate $500,000 to former U.S. Rep. Rick Hill’s gubernatorial campaign and $32,000 for Tim Fox’s campaign for attorney general.

Fox returned the money after the 9th Circuit blocked the ruling, but Hill did not.