WHITE HOUSE - It's more than 18 months until Americans vote again for president. But the incumbent is already publicly sizing up some of his potential opponents.

President Donald Trump has his eye on the expanding list of Democrats running to beat him in next year's election.

Sen. Bernie Sanders speaks during the National Act Sen. Bernie Sanders speaks during the National Action Network Convention in New York, April 5, 2019. Sen. Bernie Sanders speaks during the National Action Network Convention in New York, April 5, 2019.

Speaking Wednesday to David Webb of the Patriot channel on the Sirius XM satellite radio platform, Trump said Senator Bernie Sanders seems to "have the most spirit" among the pack of Democrats, while former Vice President Joe Biden will likely struggle to gain the nomination.

Former Vice President Joe Biden speaks at the Bide Former Vice President Joe Biden speaks at the Biden Courage Awards, March 26, 2019, in New York. Former Vice President Joe Biden speaks at the Biden Courage Awards, March 26, 2019, in New York.

"It could be Bernie. It could be Biden. And I think Sleepy Joe is going to have a hard time. He's, you know, 1 percent Joe. I don't think he's going to make it,” Trump said.

Trump also said he'd like to run against the "mayor from Indiana," a reference to 37-year-old Pete Buttigieg, who would make history as not only the youngest but the first openly gay U.S. president.