“The crucial thing for all of you to know is Harry Reid’s got 51 votes to do the Constitutional option at the beginning of the Congress,” Udall said. “My sense is if he can’t get agreement on the other side, then he’s going to go forward.”

Changing rules with a simple majority vote is considered so controversial it is sometimes called the nuclear option. Democrats backing the maneuver have described it as the “Constitutional option.”

A bipartisan group of senators including Senate Rules Committee Chairman Charles Schumer (D-N.Y.) and Sen. Lamar Alexander (R-Tenn.), the ranking Republican member of the Rules panel, are meanwhile working on a bipartisan compromise to change filibuster rules under regular order, which requires 67 votes.