A no vote from these two senators might not be enough to offset affirmative votes from endangered red-state Democrats. And the attack on Roe may be pursued through a host of more subtle and incremental assaults, not an out-and-out declared repeal. To save Roe, Ms. Collins and Ms. Murkowski might need to wield a bigger stick. Fortunately, there’s one at hand, and wielding it at this pivotal moment might do good beyond the single issue. They could bolt their party and shift the balance of power in the Senate.

Since 1890, 21 senators have switched party affiliation during their time in office, some for matters of conscience, some to advance careers. In 2001, when the Senate was split 50-50, Jim Jeffords renounced his Republican membership to become an independent aligned with the Democrats, flipping control of the chamber. Arlen Specter, the last senator to switch party affiliations, became a Democrat again in 2009 for the same reason he became a Republican in 1965: he couldn’t get elected otherwise. Mr. Specter’s was an act of self-preservation. Ms. Collins and Ms. Murkowski have an opportunity for a principled act of national preservation.