As news spread that Michael Cohen’s plea deal had all but named President Trump as an unindicted co-conspirator in a federal crime, I thought back to a conversation I had earlier this summer in South Carolina.

A woman approached me at the end of our Need to Impeach Town Hall in Columbia, where I’d discussed the threat Donald Trump poses to our democracy. At 52, she was a lifelong Republican, but she couldn’t understand why her party’s elected leaders wouldn’t stand up to Mr. Trump.

“We’re...