Donald Trump said he would rather run against Michael Bloomberg than new Democratic frontrunner Bernie Sanders – and suggested the US military punish an Army officer who testified against him during House Democrats' impeachment inquiry.

As the 2020 race heats up, the president said he wants to face Mr Sanders because "Bernie has real followers," taking a backhanded swipe at the former New York City mayor before taking a direct one when he accused Mr Bloomberg of "buying his way in" to the Democratic nominating process.

During yet another impromptu Oval Office question-and-answer session following a military veterans event, Mr Trump called Mr Bloomberg "a lightweight" and "pathetic."

Polls show Mr Trump trailing both men in hypothetical one-on-one general election race.

The comment came after the president took to Twitter earlier on Tuesday to call Mr Bloomberg a "total racist" over comments the former mayor made while still in office about his "stop-and-frisk" policy. Audio and video recordings surfaced this week with Mr Bloomberg defending the policy and his order to send more NYPD officers into minority communities, saying he did so because that's where most of the city's crime was at the time.

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Meantime, Mr Trump sought further retribution against Army Lt Col Alexander Vindman, the National Security Council's top Ukraine adviser who had implicated Mr Trump in a quid pro quo to smear the president's political rivals.

Mr Vindman was "escorted out of the White House" on Friday after Mr Trump said hours earlier he was "not happy" with the National Security Council's director of European affairs and top Ukraine expert.

The decorated Iraq veteran's twin brother, Yevgeny Vindman, who was not involved in the impeachment hearings, was also fired from his role at the NSC.

On Tuesday, the commander in chief said military officials could "handle him any way they want." But moments later, he was asked to clarify if he was suggesting disciplinary action against the Army officer.

"They would I imagine, look at that," Mr Trump said, alleging that Mr Vindman "reported a false call" when he testified in the House impeachment inquiry and said he found Mr Trump's 25 July call with Ukraine's new president inappropriate and alarming.

During the call, Mr Trump told the Ukrainian leader to "do us a favour though" and investigate US Democratic politicians, including the Bidens, after they had just talked about a massive military aid package. The senate acquitted Mr Trump on the House's charge that amounted to an abuse of power.

Pentagon leaders could soon find themselves crossways with the commander in chief. They have said Mr Vindman will merely get a new assignment.