“They are worried that Mr. Rubio, a son of Cuban immigrants, could win over Hispanic voters and that he would offer a sharp generational contrast.”

However…the DNC believes:

“[Rubio is still] peddling a tired playbook of policies that endanger our country, hurt the middle class, and stifle the American dream.”

Source: Prospect of Hillary Clinton-Marco Rubio Matchup Unnerves Democrats – NYTimes.com