A rare opposition party that has been allowed to run in the upcoming Russian parliamentary election could be taken off the ballot days before the vote.

The Parnas party led by former Prime Minister Mikhail Kasyanov said in a statement on Friday that it has received a lawsuit filed by another party with the Russian Supreme Court. The little known party called Civic Platform accuses Parnas of extremism and enticing hatred against supporters of President Vladimir Putin.

Parnas is one of two opposition parties that have openly criticized the Kremlin during the campaign ahead of the Sept. 18 vote.

Parnas said the lawsuit is due to be heard next week and could end up striking it off the ballot.