Democratic U.S. Rep. Keith Ellison has signed on to a lawsuit that challenges the constitutionality of the U.S. Senate filibuster.

Common Cause, a nonpartisan advocacy group focused on making political institutions more open and accountable, filed the suit against the U.S. Senate in the U.S. District Court on Monday. Ellison and three other Democratic U.S. House members are among those named as plaintiffs.

The suit comes amid growing partisan gridlock that's left Democrats in both the House and Senate complaining that Senate Republicans have gummed up the legislative process by abusing the filibuster to slow, and even block, legislation. The Senate's filibuster requires a 60-vote threshold for bills and nominations that often have support from a majority of its members.

Recent legislative efforts to weaken the filibuster have fallen short, in part, because changing Senate rules requires 67 votes, seven more than the filibuster threshold itself. So, now the challenge goes to court.

"Ideological purists in both parties appear to have taken control of Congress and of the national dialogue," Common Cause president and CEO Robert Edgar said in a statement.

"Voices of moderation are conciliation are being drowned out on the airwaves and inside the Capitol; critical problems are going unaddressed," said Edgar, a former U.S. House member from Pennsylvania. "Things are especially bad in the Senate. Both parties have figured out that the minority, currently the Republicans, can use the filibuster to pretty much shut the place down."

Here's a copy of the complaint filed:

Common Cause Complaint