His comments by the president's eldest son came after CBS host Gayle King asked him to name his "favorite" Democrat. At first, Trump Jr. hesitated to answer but he eventually called out Romney as King ended the segment.
"Mitt Romney, Gayle, Mitt Romney's my favorite Democrat," Trump Jr. said. "No," King responded before wagging her finger at him.
Trump Jr.'s jab came after a series of barbs between President Trump and Romney, the Republican presidential nominee in 2012.
In October, Romney accused the president of displaying "appalling" behavior by calling for China and Ukraine to investigate former Vice President Joe Biden and his son Hunter.
"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 tweeted.
He added: "By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."
Rudy Giuliani, the president's attorney, responded to Romney by accusing him of being "bitter" about Trump. "Look, Mitt, Trump did what you couldn't do. Trump has an ability to relate to people -- you don't," GIuliani said at the end of September.
Trump, for his part, has called Romney a "pompous a--." "If Mitt worked this hard on Obama, he could have won. Sadly, he choked!" he tweeted.