WASHINGTON—The Supreme Court refused Monday to reconsider its Citizens United decision lifting restrictions on corporate and union political expenditures, voting 5-4 to invalidate such limits in Montana's century-old Corrupt Practices Act.

The Montana Supreme Court had upheld the state law, citing language in Citizens United suggesting that independent political expenditures might be limited if evidence showed they corrupted the democratic process. Montana adopted its regulations in a 1912 voter initiative sparked by outrage...