Senator John Cornyn was not consoled when President Trump professed on social media that the senior Republican from Texas would “remain my friend for a long time to come” after the president had enthusiastically endorsed the man who defeated Mr. Cornyn, ending his Senate career. “If that’s the way friends treat you, you wonder about his enemies,” Mr. Cornyn said this week in his first extensive interview since his loss two weeks ago to Attorney General Ken Paxton of Texas, an opponent Mr. Cornyn labeled corrupt and unfit for the Senate. Mr. Cornyn said he had come to terms with...