Khizr Khan is of course free to denounce Trump on a political platform, but then he becomes part of the political platform for better or for worse.
If we had a media, they would point out that Khizr Khan is an immigration lawyer who specializes in immigration from Muslim countries and opposes Trump's plan to limit immigration from Muslim countries. Perhaps a conflict of interest?
Khizr Khan Deletes Law Firm Website that Specialized in Muslim Immigration
Khan has also said that the nasty Donald Trump has a "black soul" and "blackness of character", using racist imagery that is not OK to African Americans.
Khan claims that Trump does not understand the Constitution, but even as an immigration lawyer does not mention that the Immigration Act of 1952 gives the President the power to exclude "any class of people". And Khan also has a history of writing favorably about Sharia law and implying that it will supersede the Constitution.
Quote: Khizr Khan ... is an attorney who has previously written in a law journal about Islamic law. He specifically wrote about the purity of the Quran and the Sunnah over all other texts and interpretations.
All of this is fair game in a political debate. Khan has the right to say whatever he wants, and to be questioned for it.