Republicans who have tried to walk a careful line with President Donald Trump are having a rough stretch in primary season.

Louisiana Sen. Bill Cassidy and Texas Sen. John Cornyn both lost to Trump-backed challengers despite years of attempting, in different ways, to accommodate the president politically.

But accommodation is rarely enough with Trump. Republican senators hoping selective loyalty will protect them should pay attention to what happened to Cassidy and Cornyn: If Trump decides you are insufficiently loyal, you are replaceable no matter how often you vote with him.

Republican senators deciding whether and when to break with Trump should take that lesson seriously. The time to do it is when you still hold power, not once you’ve become a lame duck. The former requires courage. The latter does not.

It's easier to find principles once your political future is gone

Republicans who lose primaries to Trump-backed challengers often discover a new willingness to oppose him once the political consequences no longer matter.

One example is Cassidy, who visibly........

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