President Donald Trump, in an interview with Telemundo set to air Thursday night, said top Democratic presidential contenders Bernie Sanders and Joe Biden look "tired crazy" and "exhausted."

Asked by NBC News and Telemundo anchor Jose Diaz-Balart about who he would prefer as an adversary in 2020, Trump mentioned Sanders, the Vermont senator, and Biden, the former vice president. Both have found themselves at the top of early Democratic primary polling.

"Bernie looks like he's had it," Trump said. "Bernie looks crazy, but he always did. But he looks like a tired crazy right now."

On Biden, Trump said, "he looks like he's just exhausted."

"I don't know what happened to him but he is exhausted and he doesn't do any work, he's not working," Trump added. "One thing I found out about this job, to do it right, and I've done it right — you have to work hard."

Sanders and Biden will be standing next to each other on the second night of NBC News' Democratic debate next week. Trump, 72, has repeatedly taken aim at the stamina of his older opponents, Sanders, who is 77, and Biden, who is 76.

In an interview with ABC News last week, Trump said he "probably" averages "four or five hours" of sleep every night. Trump has come under fire for his work schedule in recent months after Axios reported in February that nearly 60 percent of Trump's schedule is unstructured "Executive Time." The president often spends the first five hours of his day watching TV, reading newspapers and phoning advisers to discuss news items, the report said.