Nicole Hemmer, a historian at Columbia University who studies the conservative media, said people often mistakenly refer to the Fox News-talk radio world as an “echo chamber” of opinion when in fact it is more like “an interconnected set of authorities.”

“Sean Hannity talks about John Solomon,” she said, “and then that gets picked up on Rush and Levin.” The effect, she added, is that his reporting carries weight with conservative audiences. “That gives it an authority when they’re hearing it from multiple sources every day.”

When Mr. Solomon appears on television and the radio, Mr. Hannity and other conservative hosts often identify him as an investigative reporter and cite his decades of experience at news organizations like The A.P. But his critics see this as a sleight of hand to give his writing a veneer of nonpartisan objectivity.

“Part of the issue is that for years he was identified with the mainstream media,” said James Manley, a former aide to Harry Reid, the former Senate majority leader who tangled with Mr. Solomon in the 2000s over stories insinuating Mr. Reid had benefited inappropriately from his office. The Columbia Journalism Review later singled out Mr. Solomon’s reporting, saying it was “much ado about very little.”

An examination of Mr. Solomon’s reporting methods and interviews with people who have worked with him during his decades-long career in Washington show that his techniques blur the boundaries meant to keep journalist-source relationships at an arm’s length. And for some of his biggest stories on Ukraine, he has relied on a prosecutor with a history of making inconsistent statements who is now under criminal investigation.

At one point while he was employed as a columnist for The Hill and publishing regular pieces favorable to the president, Mr. Solomon discussed with colleagues a proposal to create a transparency office in the Trump White House. Some colleagues believed he might have wanted to run this office, according to a person with direct knowledge of the situation .

(Unlike Fox, The Hill put a disclaimer over Mr. Solomon’s writing indicating that it was opinion starting in May of 2018 after complaints from colleagues about what they saw as one-sidedness in his work, The Post reported at the time.)