The biggest question in Washington for the next two years isn't about a single policy or whether President Joe Biden will expose himself to a news conference. It's whether Democrats use their narrow Senate majority to kill the legislative filibuster rule requiring 60 votes in order to ram a radical agenda into law with a mere 50 votes plus Vice President Kamala Harris.

Two Democrats — Kyrsten Sinema of Arizona and Joe Manchin of West Virginia — promised at the start of the year that they wouldn't vote to do so. But progressive and news media pressure is building on the pair to renege on their pledges, as legislation passed by the House piles up at the Senate door. Democratic Senate leaders are vowing that they'll find a way to evade the filibuster.

Republicans can see these signs, and last week Minority Leader Mitch McConnell made clear what would happen if they do kill the filibuster. It won't be pretty.

He then explained what that could mean in practice if Republicans responded by withdrawing the unanimous consent required for the Senate to function: "I want our colleagues to imagine a world where every single task, every one of them, requires a physical quorum — which, by the way, the Vice President does not count in determining a quorum."

That's right. A quorum without unanimous consent is 51 senators, and there are only 50 Democrats. If Republicans kept their nerve in opposition, Democrats couldn't confirm nominees or vote on legislation. The Nancy Pelosi-Joe Biden agenda couldn't move any more than if there were a filibuster.

Democrats may think this is a bluff, or that the public would revolt if Republicans ground Senate business to a halt. But are they willing to take that bet?