President Trump referred to Mitt Romney as a "pompous ass" on Saturday in response to the Republican senator's criticism of Trump for his "brazen and unprecedented appeal to China and to Ukraine" to investigate the Bidens.

Why it matters: Romney, who criticized the Trump-Ukraine allegations after reading the whistleblower complaint, is one of the first Republican senators to denounce Trump's calls for China or Ukraine to investigate one of his top 2020 rivals.

Background: Trump told reporters this week that China should launch an investigation into Joe and Hunter Biden, though he noted he hasn't yet asked Chinese President Xi Jinping to do so. Trump argued on Saturday that his previous statement "pertained to corruption, not politics."

Trump is currently facing a House impeachment inquiry following a whistleblower complaint that alleges the president used "the power of his office to solicit interference from a foreign country in the 2020 election."

following a whistleblower complaint that alleges the president used "the power of his office to solicit interference from a foreign country in the 2020 election." Officials named in the whistleblower complaint drafted a statement in August committing Ukrainian President Volodymyr Zelensky to investigate the Ukrainian energy company for which Hunter Biden previously served on the board.

Go deeper ... Senate's new maverick Republican: Mitt Romney and the whistleblower complaint