It’s going to be tough for him to win Florida. Jewish folk? Non-Cuban Hispanics? Second generation Cubans who never experienced the horrors of Communism? No Florida, no White House. That’s just the way it is.
A good VP can make you look better; a bad VP pick just confirms what people feel about you all along.
No one will vote for the understudy so at best in terms of moving votes its pretty much a wash.