Washington (CNN) President Donald Trump lashed out at Republican Sen. Mitt Romney of Utah Saturday after Romney rebuked him for calling on Ukraine and China to investigate the family of former Vice President Joe Biden.

In a crude attack against a sitting senator from his own party, Trump tweeted, "Mitt Romney never knew how to win. He is a pompous 'ass' who has been fighting me from the beginning, except when he begged me for my endorsement for his Senate run (I gave it to him), and when he begged me to be Secretary of State (I didn't give it to him). He is so bad for R's!"

Trump's attack comes as he faces a widening impeachment inquiry in the House of Representatives. Most congressional Republicans have remained silent and declined to speak out in opposition to the President's comments on Thursday that he wants both Ukraine and China to investigate Joe Biden and his son, Hunter. But Romney criticized the President in a statement Friday, saying, "By all appearances, the President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

There is no evidence of wrongdoing by either Joe or Hunter Biden.

Romney also said, "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." CNN has reached out to Romney's office for response to the President's tweet.

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