He is bringing this up to stir the pot. He brought up Cruz's father to stir the pot.

One would think that if you want to win you would not bring up the birther movement when people are trying to decide who to vote for. The people that never believed Obama was a citizen are likely to support him so how does this position add voters.

Fact Checkers Prove That 91% of the Things Donald Trump Says Are False
By Sarah Jones on Thu, Mar 31st, 2016 at 12:05 pm
Donald Trump, the Republican front-runner, is at the bottom of the Politifact list with a sad 9% of true or mostly true statements. Just 9% of the things Donald Trump says are mostly related to the truth.