President Donald Trump resumed his years-long vilification of Senator Mitt Romney on Saturday morning, calling the 2012 GOP presidential nominee a "pompous 'ass' " the day after Romney criticised Trump's requests that foreign leaders investigate a political opponent.

"Somebody please wake up Mitt Romney and tell him that my conversation with the Ukrainian President was a congenial and very appropriate one, and my statement on China pertained to corruption, not politics. If Mitt worked this hard on Obama, he could have won. Sadly, he choked!" Trump tweeted.

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Then, Trump continued, "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 personal attack on Romney for speaking out against him underscores why so few Republicans do so, out of fear of the president's wrath.