"Pulling a bulworth" is when a candidate starts saying what's on his mind, not caring what the public, media, or special interests think.
It's a reference to the movie "Bulworth" starring Warren Beatty. Bulworth is a senator running for re-election. Knowing that he is going to die in a few days, Bulworth stops caring what people think. He stops catering to special interests and starts saying what he actually believes. Although he often says offensive or politically incorrect things, it causes him to rise in the polls.
That candidate knows he is retiring this year. He's pulling a bulworth because he doesn't have to be re-elected.