It does not matter. If he becomes President the ban will lift because
A) He is a leader of a country that is closely allied with UK
B) He is an American president, after all.
We had a similar situation with our current Prime Minister before he got elected. He was banned to travel in a few countries including USA (for his genocide against Muslims in 2002 in Gujrat; a state with one of the worst marginalisation of minorities) but the moment he rose to become the PM, not only he received congratulatory calls from the leaders around the world but all the bans were lifted. In fact, he was welcomed in USA in a grand fashion.