The whistleblower whose complaint sparked an impeachment inquiry into President Trump had a previous “professional” relationship with one of the 2020 Democratic presidential candidates, a bombshell report reveals.

Intelligence Community Inspector General Michael Atkinson told lawmakers during a closed-door session that the whistleblower, identified as an intelligence officer, admitted he was a registered Democrat and at some point had ties to one of the candidates — a development first reported by The Washington Examiner and confirmed by Axios.

The report prompted President Trump to question the whistleblower’s credibility.

“Wow, Just Breaking: ‘The (big deal) Whistleblower had a ‘PROFESSIONAL TIE’ to 2020 Democratic Candidate.’ Washington Examiner, @ByronYork In other words, was working with someone who was potentially running against me. Why wasn’t this reported by the ICIG? A Witch Hunt Scam!” the president tweeted late Tuesday night.

The Examiner cited three sources for the revelation.

“The IG said [the whistleblower] worked or had some type of professional relationship with one of the Democratic candidates,” one source told the paper.

“The IG said the whistleblower had a professional relationship with one of the 2020 candidates,” a second source added.

“What [Atkinson] said was that the whistleblower self-disclosed that he was a registered Democrat and that he had a prior working relationship with a current 2020 Democratic presidential candidate,” a third person with knowledge of what was said told the Examiner.

Atkinson in his Aug. 26 letter to acting Director of National Intelligence Joseph Maguire noted the presence of “arguable political bias” on the whistleblower’s part, but added that “such evidence” did not change his determination that the information “appears credible.”

The complaint — alleging Trump had pushed Ukrainian President Volodymyr Zelensky to probe political foe and Democratic presidential candidate Joe Biden — prompted House Speaker Nancy Pelosi to launch the impeachment inquiry.

All three sources said Atkinson did not identify the Democratic candidate and the nature of the relationship between the two was unclear.

Fox News previously reported Atkinson had revealed the whistleblower volunteered that he was a registered Democrat and that he had a prior working relationship with a prominent Democratic politician.

But it was unknown until the Examiner’s report Tuesday that the Democratic official was running for president — a fact that prompted Trump to question the integrity of the whistleblower’s information.

“The Whistleblower’s facts have been so incorrect about my ‘no pressure’ conversation with the Ukrainian President, and now the conflict of interest and involvement with a Democrat Candidate, that he or she should be exposed and questioned properly. This is no Whistleblower,” the president tweeted.

“The Whistleblower’s lawyer is a big Democrat. The Whistleblower has ties to one of my DEMOCRAT OPPONENTS. Why does the ICIG allow this scam to continue?” Trump continued, referring to Atkinson.

The whistleblower, whose name has not been revealed, filed a complaint in August about the July 25 call between Trump and Zelensky, in which Trump sought an investigation into ex-veep Biden and son Hunter.

Trump, speaking Wednesday at a White House bill signing on government transparency, said he wanted the whistleblower identified because he doesn’t want calls with foreign leaders leaked to the press or Democrats.

“I don’t want spies in the White House,” the president said, repeating his contention that the impeachment effort was a scam.

“It is all a big con,” he said. “We have the transcript, just read the transcript.”

In response to the report, the whistleblower’s lawyer released a statement denying that his client was biased.

“Our client has never worked for or advised a political candidate, campaign, or party. Our client has spent their entire government career in apolitical, civil servant positions in the Executive Branch,” attorney ­Andrew Bakaj wrote.

“In these positions our client has come into contact with presidential candidates from both parties in their roles as elected officials — not as candidates,” he continued, adding that IG ­Atkinson has stood by the whistleblower’s account.

“The ICIG concluded — as is well known — that the complaint was both urgent and credible. Finally, the whistleblower is not the story. To date, virtually every substantive allegation has been confirmed by other sources,” Bakaj concluded.

The statement was also signed by Mark Zaid, the lawyer for a second whistleblower to come forward about Trump’s call.

Trump’s lawyers on Tuesday sent a letter to House Democratic leaders declaring a full halt to cooperation with what the administration termed an “illegitimate” probe, in part because a formal vote has not taken place.

House Speaker Nancy Pelosi responded by telling the president he was not above the law.

“The White House should be warned that continued efforts to hide the truth of the president’s abuse of power from the American people will be regarded as further evidence of obstruction,” Pelosi said.

“Mr. President, you are not above the law. You will be held accountable.”

GOP Sen. Lindsey Graham said he doesn’t think there was anything impeachable in the transcript of the July call, in which Trump asked the Ukrainian president for “a favor” at a time that military aid, which was later released, was being withheld.