He went to the border, he said in an interview on Thursday, because he thought reports of children being separated from their families might be fake news: “I just couldn’t envision that the administration would actually take asylum seekers, those fleeing persecution, and deliberately inflict trauma on their children.”

It is undoubtedly not lost on him that the ensuing publicity is good for his career; Mr. Merkley also acknowledged that he was “exploring the possibility” of a 2020 presidential bid. (He has previously said he was “keeping the options open.”) Asked whether he would stay out of the race should Ms. Warren or Mr. Sanders become candidates, he leaned back in his chair and said, “Not necessarily.”

Mr. Merkley’s unsuccessful foray to the shelter, in Brownsville, has not been without controversy. While in Texas, he also visited a processing center at the border station in McAllen, and came back describing “hundreds of children locked up in cages there” — an assertion that helped earn him a “Three Pinocchios” rating in a fact-checking report by The Washington Post.

That rating was later downgraded to “Two Pinocchios” after lawmakers and journalists corroborated Mr. Merkley’s claim; the remaining Pinocchios involved his inaccurate assertion that outsiders are barred from visiting the shelter when visits are, in fact, allowed with two weeks’ notice. The White House, for its part, accused Mr. Merkley of “irresponsibly spreading blatant lies.”

Mr. Merkley, who has never met the president, is steamed about that. “I know I took a lot of heat for using that term, but that’s what they look like,” he said, referring to the “cages” comment.