The Republicans just need to put a candidate up who can run an effective campaign and not say stupid shit like Romney did in 2012. Romney ran the worst campaign possible in a very favorable year to oppose Obama.
I say this because either way they're going to face a highly beatable Democrat.
Clinton is easily one of the absolute worst candidates to run in the modern era. She's corrupt, has more baggage than a hotel, most Democrats don't even like her, she will do nothing to inspire the black vote, middle class white people don't like her, and she's absolutely awful in debates. This was all true in 2008 and that's why Obama so easily over took her.
On the other hand, if the D's elect Bonkers Sanders, it's even easier. The guy is a fringe left wing extremist who excites the hardcore socialist zealots but no one else. He'd be exposed the minute he had to debate these ideas of his and start putting price tags next to them.
A Rubio/Fiorina ticket would win easily IMO