When rules constrain Senate partisanship, voters in swing states can view candidates as independent figures rather than partisan foot-soldiers. Now that changes: To elect even a 50-50 Senate with a Democratic President could be to authorize much of the progressive agenda.

This will be a hard perception to shake on the 2022 campaign trail for Maggie Hassan in New Hampshire and Michael Bennet in Colorado, both of whom also signed Mr. Coons’s 2017 letter, and Catherine Cortez Masto of Nevada. Raphael Warnock of Georgia benefited in his 2021 runoff election from Mr. Manchin’s 2020 promise that he wouldn’t eliminate the filibuster in a 50-50 Senate. Now voters need to keep in mind that Mr. Manchin’s commitment means nothing if Democrats pick up two seats in 2022.

As for Republicans, the next GOP Senate majority will now be under much more pressure to eliminate the filibuster as a pre-emptive procedural strike. Populist Senators will point to this week’s vote to say that Democrats are planning to change the rules as soon as they get back into power. GOP leader Mitch McConnell and other institutionalists will have more trouble talking them down.