Bill Kristol, writing last month for The Bulwark:

The Democratic polling and messaging firm Blueprint recently tested the effectiveness of several closing messages for the Harris campaign. (This was before Kelly’s new remarks.) Here’s one message the group put before voters:

Donald Trump doesn’t have the character it takes to be president. He’s erratic and can’t control himself. He denied the results of an election just because he lost and is a threat to the fundamental American principle of democracy. He instigated a riot at the Capitol that left three police officers dead.

This general (and true) statement barely moved the needle on voters’ preferences. It presumably simply sounds like a reiteration of things voters have heard before.

What did move the needle was this message:

Nearly half of Donald Trump’s Cabinet have refused to endorse him. When Trump learned during the Capitol riot that his supporters were threatening to kill his own vice president, he said ‘so what?’ and refused to do anything to assure the vice president was safe. Republican governors, senators, and House members have all said the same thing: We can’t give Trump another four years as president.

As soon as the message turned from an abstract argument against Trump into an unambiguous case that Trump’s own former allies were making against him, it became the single most persuasive line tested by Blueprint. It was stronger even than abortion rights and Social Security. In other words, hearing about Trump’s unfitness from people who worked with him, and from Republicans one would expect to defend him, seems to make a difference.