Oprah appeals to the more liberal minded people. But if you want Trump gone, we need someone that will appeal to the moderate and conservative-light people. Obviously the liberals are going to vote for the Democrat whoever it is. But the moderates and conservative-light people need to be won over.
Alternatively, and probably more easily; get someone that actually excites the liberal base—get the non-voters to go out and vote. Going right isn’t always, or even usually, the answer.