President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE said in a tweet early Sunday that Democrats are “lucky” not to have any “Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE types” after the Republican senator said Trump’s call for China and Ukraine to investigate Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE was “wrong and appalling.”

"The Democrats are lucky that they don’t have any Mitt Romney types," Trump tweeted. "They may be lousy politicians, with really bad policies (Open Borders, Sanctuary Cities etc.), but they stick together!"

The Democrats are lucky that they don’t have any Mitt Romney types. They may be lousy politicians, with really bad policies (Open Borders, Sanctuary Cities etc.), but they stick together! — Donald J. Trump (@realDonaldTrump) October 6, 2019

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"When the only American citizen President Trump singles out for China’s investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated," Romney, a former GOP presidential nominee and current Utah senator, tweeted Friday.

"By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling," he added.

Trump on Saturday called for Romney to be impeached – though a senator can't be impeached – and argued that Republican voters in Utah made a "mistake" in nominating him for the Senate.

Also on Saturday, Trump lashed out at Romney and called him a "pompous 'ass.' ”

"Somebody please wake up Mitt Romney and tell him that my conversation with the Ukrainian President was a congenial and very appropriate one," Trump asserted in a tweet.

Read more from The Hill:

Key Republicans split with Trump on Biden investigation push