President Donald Trump was back on a Twitter tirade Thursday — this time, responding to reports that he is being investigated for obstruction of justice.

Late Wednesday came reports that an investigation of Russian meddling in last year’s presidential election has expanded to look at Trump for possibly impeding that probe.

Trump hit back on social media, tweeting two responses about an hour apart.

“They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice” came first, followed by tweet that used, as he has before, the phrase “witch hunt” to describe the investigation.

“You are witnessing the single greatest WITCH HUNT in American political history – led by some very bad and conflicted people!” he said, referring to the Department of Justice, led by people he appointed.

POLL: Most in U.S. think Trump meddled in Russia probe

The expansion of special counsel Robert Mueller’s investigation would represent the clearest legal threat to date for Trump, who has long maintained that he is not personally under investigation — and who reportedly pressured former FBI Director James Comey to say so publicly.

Robert Mueller | Saul Loeb/AFP/Getty Images

The Washington Post, citing unnamed sources, said the investigation into the president’s own conduct began shortly after Trump fired Comey on May 9. That prompted Deputy Attorney General Rod Rosenstein to appoint Robert Mueller — himself a former FBI director — as special counsel the following week.

The appointment of Mueller — who was FBI chief under both Democratic and Republican presidents, and held that post during the 9/11 atttacks — was generally praised by members of both parties, and confidence in his professionalism was professed again during Tuesday’s testimony by Attorney General Jeff Sessions before the Senate Select Committee on Intelligence.

In recent days, however, there were attempts by some more ardent Trump supporters to insinuate that Mueller was biased and that Trump was thinking of firing him.

As Mueller built his legal team, some of those Trump allies raised questions about the former FBI director’s impartiality, suggesting he cannot be trusted to lead the probe.

Trump friend Chris Ruddy, the CEO of Newsmax, went so far as to suggest the president was already thinking about “terminating” Mueller.

“I think he’s considering perhaps terminating the special counsel,” Ruddy said in an interview with Judy Woodruff of “PBS NewsHour.”

But House Speaker Paul Ryan, R-Wisconsin, disagreed, saying the the White House and Trump should let the special counsel’s investigation continue, and await vindication.

Ryan told reporters Tuesday: “The best advice would be to let Robert Mueller do his job. … I know Bob Mueller. I have confidence in Bob Mueller.”

Rep. Adam Schiff, the top Democrat on the House Intelligence committee, said such a move would “be the last straw” for many in Congress and would have “echoes of Watergate,” when President Richard Nixon dismissed special prosecutor Archibald Cox over Cox’s subpoenas for White House tapes.