• Donald Trump railed against fellow billionaire Mark Cuban, who has been vigorously campaigning for his Democratic opponent, Vice President Kamala Harris.
• Trump's insults come several days after Cuban mocked the Republican presidential nominee's tariff proposals at a Harris campaign rally.
• The animosity between two billionaires is a stark pivot from 2015 when Cuban praised Trump as the "best thing to happen to politics in a long time."

Former President Donald Trump on Saturday railed against fellow billionaire Mark Cuban, who has been vigorously campaigning for his Democratic opponent, Vice President Kamala Harris.

"Mark Cuban is a Loser," Trump wrote in a Saturday Truth Social post. "Wouldn't take his phone calls anymore while at the White House, and he went rogue. A weak and pathetic 'bully,' he's got nothing going. Really low clubhead speed, a total non-athlete!"

Trump's insults come several days after Cuban mocked the Republican presidential nominee's tariff proposals at a Harris campaign rally.

"Let me just tell you, this man has so little understanding of tariffs, he thinks that China pays for them," Cuban said of Trump at a Harris rally in Wisconsin on Thursday.

Trump has proposed a 20% universal tariff on all imports and an even higher 60% rate for Chinese imports. The plan has faced criticism from economists, who note that U.S. importers and consumers would ultimately shoulder the cost burden of such a hardline approach.

The animosity between two billionaires is a stark pivot from 2015 when Cuban praised Trump as the "best thing to happen to politics in a long time," in a "Fox & Friends" interview.

Trump in turn thanked him for the kind words.

Cuban ended up endorsing former Secretary of State Hillary Clinton in the 2016 election. And in the years since, his once-friendly relationship with Trump has only grown icier.

This election cycle, Cuban has become a regular Harris campaign surrogate as the vice president works to frame herself as a pro-growth capitalist.

The former "Shark Tank" host could have his sights on a government position of his own, repeatedly floating the idea of replacing Securities and Exchange Commission Chair Gary Gensler, for example.

Meanwhile, Trump has enlisted Elon Musk as his own mega-billionaire campaign surrogate, fueling another rivalry with Cuban.

"You can be one of the world's greatest entrepreneurs literally of the last century and be a f---ing troll at the same time," Cuban said of Musk in a Friday interview on "Real Time With Bill Maher."