WASHINGTON — A long-shot lawsuit challenging the Senate filibuster rules, in part over a contentious immigration issue, was tossed out Friday by a federal judge.

In a 47-page ruling, U.S. District Judge Emmet G. Sullivan acknowledged that the “filibuster rule is an important and controversial issue . . . as in recent years, even the mere threat of a filibuster is powerful enough to completely forestall legislative action.”

Nonetheless, Sullivan added that he was “powerless to address this issue” for several reasons.

“Reaching the merits of this case would require an invasion into internal Senate processes at the heart of the Senate’s constitutional prerogatives as a House of Congress, and would thus express a lack of respect for the Senate as a coordinate branch of government,” Sullivan wrote.

The government watchdog group Common Cause filed the lawsuit on behalf of itself and several members of the House of Representatives, who contended Senate Republicans had used the filibuster to block campaign finance reform legislation. Joining the suit were three individuals who said they would have benefited from the so-called DREAM Act had it not been blocked by a filibuster.