WaPo endorsing Harris would have changed exactly 0 minds. Everyone that would have read it already agrees with them. But it would have made them feel good about their existing world view.
Them _not_ endorsing _will_ change minds. There are people that read Washington Post that would take that as a sign that not even their trusted left-leaning paper is 100% comfortable with the candidate they should have endorsed, so maybe there's something there they should have hesitancy about too.
Them _not_ endorsing _will_ change minds. There are people that read Washington Post that would take that as a sign that not even their trusted left-leaning paper is 100% comfortable with the candidate they should have endorsed, so maybe there's something there they should have hesitancy about too.