The incumbent President thinks so. Obama says he's had policy differences with previous Republican presidents and candidates, but never thought they could not function as President - simply disagreeing with their policies. But Trump is a different matter.
Also today, Richard Hanna became the first Republican member of Congress to publicly say he would vote for Hillary Clinton, citing Trump's comments about the Khan family as the deciding factor - not for a third party or write-in candidate, but for Clinton!
Obama questions why the Republicans still support the Trump's candidacy. Should they (and could they?) drop him for somebody else at this stage?
See
http://www.bbc.co.uk/news/world-us-canada-36958126 for more details.