In the current head-to-head Presidential polls, Sanders is beating every GOP candidate but Rubio.
Rubio is the one candidate that the Democrats feared - but after being savaged by Christie, a poor showing in the New Hampshire debates and being attacked by Cruz and Trump for the next 5 months, all these candidates will be perceived as "damaged goods" by the time one makes it to the general election.
Given the level of hostility in the Republican primaries, it makes little difference if Trump, Cruz, Rubio or Bush win the nomination - the supporters of the other candidates are already so polarized that many will probably choose to stay home in November!
Clinton or even Sanders could win in a walk, appoint the next chief justice and perhaps win enough seats on their "coattails" to regain control the Senate!