Democrats fear a Rubio ticket?

NY Times:

A Clinton-Rubio Matchup Unnerves Many Democrats

They are worried that Marco Rubio, a son of Cuban immigrants, could win over Hispanics and offer a sharp generational contrast.

Rubio is an articulate attractive candidate, and he certainly qualifies as from a different generation than Hillary Clinton.  Whether he would still be a threat to the Democrats will depend on how effective their smear machine is.  He does still have to get the nomination and he will be up against some strong opposition.