It’s a mistake to doubt the sincerity of Ms. Collins’s and Ms. Murkowski’s dedication to questions of women’s health. Ms. Collins was one of only three Republican senators to vote against the Partial-Birth Abortion Ban Act. Last year, she and Ms. Murkowski broke ranks to cast the deciding votes against defunding Planned Parenthood. And this year, the two senators again voted with the majority of Senate Democrats to oppose a ban of abortion after 20 weeks. Both have been solid defenders of family planning and birth control. Both have earned high ratings with pro-choice groups — Ms. Collins has been rated higher than 90 percent by groups like NARAL in multiple years. To cave on this particular issue would be a tragic abrogation of personal, as well as political, conviction. Which is why the Supreme Court battle may at last compel both women to abandon the party they should have abandoned already.

A Senate no longer in the Republican grip could rescue women’s reproductive rights on multiple fronts: maternity coverage, access to contraception, teen-pregnancy prevention, breast-cancer treatment. It could protect us from planned eviscerations of Social Security and Medicare, and stop the gutting of Obamacare, the greenlighting of partisan gerrymandering, and the enshrining of an ethically shameless kleptocracy. It would thwart the authoritarian dream of seamless one-party rule across all three branches of government. Even a threat of leaving would send a salutary shock through the party.

Maine Senator (and Congresswoman) Margaret Chase Smith, Ms. Collins’ political foremother and idol, often broke ranks with her party — to defend, for instance, F.D.R.’s New Deal legislation from conservative attacks. On June 1, 1950, she became one of the first members of Congress to denounce the anti-Communist witch hunt of fellow Republican Senator Joe McCarthy. She began her Declaration of Conscience speech: “I would like to speak briefly and simply about a serious national condition. It is a national feeling of fear and frustration that could result in national suicide and the end of everything that we Americans hold dear.” She did not want “to see the Republican Party ride to political victory on the Four Horsemen of Calumny — Fear, Ignorance, Bigotry and Smear.”

After six moderate Republican senators signed the declaration, Mr. McCarthy labeled them, in Trumpian fashion, “Snow White and the Six Dwarves.” He had Ms. Smith removed from her post on the Permanent Subcommittee on Investigations (replacing her with Richard Nixon) and lavished support on her challenger in the next election (she won anyway).

Ms. Smith could at least appeal to Republican legislators willing to put reason, compassion, and country ahead of party. Trumpist Republicans have no room for such niceties — they care only about winning. Which is why only losing the Senate can get their attention, and slow the party’s extremism.

We are at another watershed moment that could “result in national suicide and the end to everything we Americans hold dear.” As the damage and outrages pile up, it’s important to remember that Ms. Collins and Ms. Murkowski are enabling them with their affiliation, even when they dissent. They could halt the whole charade, and alter the course of history, with a press release.