Over 80% of Sanders supporters have said that they'll vote for Clinton if she's the nominee. In reality, the number is almost certainly larger than that since you have a certain number of people who claim they won't for polling purposes ("stick it to her!") but, when faced with a November ballot, would rather take the sour medicine than risk the fever.
Trump will probably get fewer Republican votes since there are more people stridently opposed to his candidacy and view him as destroying the Republican party. Say what you will about Clinton/Sanders but you don't have respected voices from the left saying that their win would ruin the party as we know it. That said, he'll still probably pick up 75%+ of the GOP vote.
Trump's big problem is that the only demographic that supports him over Clinton right now is white males. And there's nowhere near enough white males in the US to win an election when you're losing all the other groups (and places where it's mainly white males are electorally insignificant).