WASHINGTON — Not long ago, they called him “too extreme,” “aggressively and dangerously wrong” and “downright dangerous.” They called him “nutty,” “reckless” and “far outside the mainstream.”

Now they would like to call him their star witness.

Suddenly, John R. Bolton, the conservative war hawk and favorite villain of the left, is the toast of Senate Democrats, the last, best hope to prove their abuse-of-power case against President Trump. Democrats who once excoriated him are trumpeting his credibility as they seek his testimony in Mr. Trump’s impeachment trial.

On the other side of the aisle, some of Mr. Bolton’s longtime Republican friends are just as abruptly tossing him to the curb, painting him as a disgruntled former adviser who just wants to sell books. Some of the same senators who allied with him, promoted his career, consulted with him on foreign affairs and took his political action committee money are going along as he is painted as “a tool for the radical Dems and the deep state,” as he was termed on one of the Fox News channels, part of the network where he worked for 11 years.

“It’s a totally upside-down world,” said Senator Chris Van Hollen, a Democrat from Maryland who two years ago denounced Mr. Bolton’s “history of warmongering” when he was appointed Mr. Trump’s national security adviser. “But what we should all agree on is we want to get to the truth of the matter about the impeachment charges and we should accept his testimony under penalty of perjury.”