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In the history of impeachment inquiries, members of the president’s own party who criticized him played a big role. They were often held up as voices of conscience whose criticism underscored the president’s wrongdoing.

Senator Howard Baker, the Tennessee Republican who (eventually) turned on Richard Nixon, became a central Watergate figure. Senator Joe Lieberman, the Connecticut Democrat who harshly criticized Bill Clinton (while still opposing impeachment), later became a vice-presidential nominee partly as a result.

So far, the impeachment inquiry into President Trump has featured few Republican voices of conscience. The closest is arguably Senator Mitt Romney of Utah. House Democrats missed an opportunity to add to the list when they didn’t ask any of the conservative legal experts appalled by Trump’s behavior to testify earlier this month.