“It’s uncomfortable not having endorsed the Republican nominee, I have to say, but I can’t at this point,” Mr. Flake said on MSNBC’s “Morning Joe.”

“I hope to be able to support the nominee. I certainly can’t right now,” Mr. Flake said.

Even Republican leaders who have said they’ll support Mr. Trump, such as House Speaker Paul D. Ryan and Senate Majority Leader Mitch McConnell, have distanced themselves from Mr. Trump’s attacks on Judge Gonzalo Curiel, who is overseeing one of the cases alleging fraud against Trump University.