Donald Trump sent a senior Pentagon official charged with overseeing the military's vast policy apparatus on his way Wednesday, the latest senior official to be ousted from or leave the administration - and possibly setting up an election-year showdown on a slew of issues with Senate Democrats.

John Rood's departure comes after White House and other administration officials had expressed displeasure with him for some time, according to several media outlets. But it also removes from the administration another senior official - as undersecretary of defense for policy, he was the Defense Department's No. 3 civilian official - who was at odds with the White House on a military aid package for Ukraine.

Mr Rood last summer told lawmakers that the Eastern European country had made enough progress on fighting corruption to deserve a massive military aid package from Washington. But the White House, at the president's direction, soon froze that aid as it pressed senior Ukrainian leaders to publicly state they were investigating top US Democrats, including former Vice President Joe Biden, then the party's frontrunner for its presidential nomination.

The matter became the basis for House Democrats' impeachment inquiry, and they late last year impeached Trump on an article of abuse of power stemming from his actions towards Kyiv.

The ouster was first reported by CNN, with the president confirming Mr Rood's departure in a morning tweet from Las Vegas, where he is spending his nights during a weeklong West Coast fundraising and political rally swing that ends Friday night.

The next move is all Mr Trump's.

He could opt to nominate a permanent replacement to lead the Pentagon's policy efforts. Or he could simply install another acting official, who could fulfill the position's duties on a temporary basis - and then do so again after that individual's time would expire.

The president early last year signalled he would opt less for Senate confirmation hearings, which allow Senate Democrats to ask pointed questions - which sometimes expose his administration's flawed processes and unmet policy goals, and are played and replayed on the cable new programs he watches religiously.

"I have 'acting.' And my 'actings' are doing really great. ... Mick Mulvaney is doing great as [acting White House] chief of staff," the president said on 6 January 2019, as he left the White House for a border security summit with senior aides at Camp David. "But I sort of like 'acting.'"

"It gives me more flexibility. Do you understand that?" he asked reporters in a rhetorical question he has never fully answered nor explained.

"I like 'acting,'" Mr Trump said then. "So we have a few that are acting. ... If you look at my Cabinet, we have a fantastic Cabinet. Really good."

Democratic lawmakers, echoed by legal and government experts, have called both his turnover rate among senior officials both unprecedented and concerning.