After 30 years on the Supreme Court, Justice Anthony M. Kennedy is stepping down. He was for the past decade or so the “swing vote” — the justice whose vote was up for grabs and to whom litigators often pitched their arguments. His opinions, his critics said, were erratic, quirky and so narrowly tailored as to give insufficient guidance to the lower courts. He was for liberals an occasional hero, as when he joined the majority that ruled same-sex marriage constitutional. While conservatives groused about his support for constitutional protection for abortions, he sided with them often, as he did this term — on decisions regarding a baker’s refusal to make a cake for a gay couple, eliminating mandatory dues for public-sector employees and upholding the president’s travel ban.

Now President Trump, whose disdain for democracy and imperious view of the presidency threatens well-established democratic norms, will pick a justice who will either join the solid four conservatives or maintain a degree of independence. One imagines the Federalist Society, which has its list of desirable justices ready to go, will play a major role in selecting the next justice.

While the protection of the filibuster is gone for the minority, the GOP enjoys only a 51 to 49 advantage in the Senate, 50 to 49 in Sen. John McCain’s absence due to his battle with brain cancer. That puts remarkable power in the hands of a few moderate Republicans and conservative Democrats, a batch of which are on the ballot in November.

It is unclear whether Republican Sens. Susan Collins, Lisa Murkowski, Jeff Flake and Bob Corker will insist on a less ideological justice. The pressure on them — from both sides — will be immense. Red-state Democrats doing their best to keep their distance from Trump without losing constituents will face a decision of immense consequence. Will they have the nerve to reject a justice akin to Neil M. Gorsuch, who has proved to be dogmatically conservative? The argument they and moderate Republicans will wrestle with is whether the court’s current “balance” should be preserved. That is a political consideration but an important one in a country in which legal issues permeate every facet of public policy and daily life.

I would offer the following suggestion to those self-identified centrists in the Senate: They should act as one and use their leverage to get a judge of excellent qualifications who will resist the urge to fall in automatically with other justices. In particular with this president, senators must insist that the new justice institutionally protect the courts and reject Trump’s view that the president is beyond the reach of criminal laws, exempt from subpoenas and able to self-pardon. These issues go to the heart of our democracy, and Americans should implore their senators to use extreme care in the confirmation process and in their final pick.

A word of warning for Republicans: The last thing the GOP can afford is to add fuel to the Democratic resistance, already operating at a fever pitch. A doctrinaire judge willing to allow the president free rein and to espouse a cramped view of individual rights will surely generate a massive backlash at the polls in November.

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