“His ideological perspective blurred his ability to recognize the operational reality of what was happening at the V.A.,” said Paul Rieckhoff, the founder of Iraq and Afghanistan Veterans of America. “The reality was that he was one of the last people to publicly recognize the gravity of the situation.”

The bill he ultimately helped write included a hard-fought $5 billion to hire more medical professionals, a provision Mr. Sanders favored. As a compromise, he agreed to a Republican provision to pay for veterans to get care outside the department. It was an approach that some Republican presidential candidates — and a veterans organization backed by the billionaires Charles G. and David H. Koch — want to expand on, but Mr. Sanders and others have long feared it as a step toward privatization and a shrinking of the agency.

“Bernie initially came out like this was a Republican attack and was extremely defensive about it,” said Dr. Sam Foote, one of the primary whistle-blowers who revealed the delays at the veterans hospital in Phoenix. He said Mr. Sanders’s “impulse is to stick up for the little guy — and the V.A. serves a lot of little guys.”

“But he is no dummy,” Dr. Foote said. “He quickly realized the V.A. was lying, and he turned right around and was all over them.”

In an interview last week, Mr. Sanders rejected the notion that he was slow to respond and lenient in his oversight, saying, “We did the very best that we could to make certain that veterans get the quality health care that they need.” Instead, he spoke of his chairmanship as a period of accomplishment, highlighted by the passage of what he called “the most comprehensive” veterans health care legislation in “many, many decades.”

But when Anderson Cooper of CNN asked him on Wednesday about why he did not act sooner to address the wait times, Mr. Sanders conceded, “We should have done better.”