a) He genuinely believes he has a shot at the leadership.
He thinks it's worth a try, even if the long-term odds are stacked against him. Even at the cost of humiliation and mockery for himself and his family. Of course this will baffle normal people, but politicians are not normal people.
He's thinking like this: if I get to be PM, I'm guaranteed to be in the history books. That's a very powerful attraction for a certain kind of wanker. It's the closest to immortality they can hope to get. It's a shame that normal people have to suffer the consequences of such sociopathic ambition, but that's representative democracy for you.