The joining states, unlike Montana, ask the court to go further and reconsider core findings in Citizens United.
They argue, for instance, it was wrong for the court to say unlimited independent expenditures rarely cause corruption or the appearance of corruption.
And other critics of the Citizens United decision who believe the court was wrong to grant corporations constitutional rights, have intervened and asked the court to reverse itself.
"There is a growing bipartisan consensus that Citizens United needs to be overturned, and Montana is leading the way," said Peter Schurman, spokesman for a group called Free Speech For People. "The Supreme Court has an opportunity to revisit Citizens United here. That is important because
there is evidence everywhere that unlimited spending in our elections creates both corruption and the appearance for corruption."
On Friday, Montana's case was given a boost when U.S. Sens. John McCain, R-Ariz., and Sheldon Whitehouse, D-D-R.I., signed on in support.
The senators argue evidence following the Citizens United decision, where millions in unregulated money has poured into presidential elections,
shows that large independent expenditures can lead to corruption.