Sen. Jeff Flake said on Sunday that it’s “becoming increasingly difficult” to imagine Donald Trump making the changes he would need to earn the Arizona Republican’s endorsement.

Specifically, Flake said that Trump will need to change both his position on things like immigration reform, partnerships with NATO and the ban on Muslims as well as his “tone and tenor,” pointing to remarks the GOP nominee has made about Mexicans being rapists that do not go over well in Arizona.