But in his letter on Monday, he offered a caveat: “Not all whistle-blowers are created equal.” An anonymous whistle-blower complaint first brought to light concern about a call between Mr. Trump and Mr. Zelensky.
In an interview on Tuesday, Mr. Johnson said it was “disappointing” that the news media had focused on his concerns about an entrenched bureaucratic class working against the president. “The real guts of my response,” he said, was not about those fears, adding that he had “nothing but the utmost respect” for Colonel Vindman.
Still, he continued, he found some of the colonel’s testimony “pretty strange.”
His Republican colleagues in the Senate in large measure have been more restrained in dealing with the president’s Ukraine policies and the ensuing investigation into them. Senator Rob Portman of Ohio, who called the president and successfully implored him to release the aid to Ukraine, has methodically kept out of the limelight ever since. Senator Lindsey Graham of South Carolina, a usual defender of Mr. Trump and the chairman of the Judiciary Committee, has steadfastly ignored entreaties from the right to conduct his own counterinvestigation, and he has tried to pass that baton to Senator Jim Risch of Idaho, the chairman of the Foreign Relations Committee, who has also demurred.
Mr. Johnson has jumped right in. He and Representative Jim Jordan of Ohio, the top Republican on the oversight panel, have traded congressional aides back and forth, the Wisconsin Republican said in a brief interview, and Mr. Jordan reached out to his office last week “to give some details” before more witnesses testified before the House Intelligence Committee.
“One thing led to the other and it was kind of a natural progression of things,” he said.
At home, Mr. Johnson has cultivated a reputation as someone who will not back down from a fight. That reputation seeped through his letter on Monday, when he freely admitted to wincing after initially being told that Mr. Trump had linked the Ukrainian aid to investigations into his political rivals, and at times professed that his memory of certain conversations was blurry at best.
Detailing a meeting he had with the president as well as with three impeachment witnesses in the Oval Office, including Gordon D. Sondland, the ambassador to the European Union, Mr. Johnson wrote that he was aware that Mr. Sondland testified that the president directed the delegation to work with his personal lawyer Rudolph W. Giuliani. But Mr. Johnson said that he had no such recollection.
“It is entirely possible he did, but because I do not work for the president,” he wrote, “if made, that comment simply didn’t register with me.”