Donald Trump. CHRIS KEANE/REUTERS Donald Trump went after Democratic presidential candidate Bernie Sanders at a Monday rally, declaring his candidacy "over."

The criticism of Sanders, an Independent senator from Vermont, started with Trump mocking Sanders for allowing protesters to interrupt one of his speeches and take his microphone away.

"That's not happening here, folks, that's never going to happen here," Trump said at a Virginia rally.

Trump then hurled an insult at Sanders that he has frequently directed at his Republican rival, Florida Sen. Marco Rubio.

"He walked by like a little boy, I'll tell you," Trump said. "It was disgusting. When I saw that, I said, 'This is going to be a leader?' But he's over now anyway, in all fairness. Nobody is showing up, and Bernie Sanders is over. He had a good run."

He then predicted that Sanders would see his crowds start to shrink at rallies, after his losses in the Nevada caucuses and South Carolina primary to rival Hillary Clinton.

"In all fairness, by far I have the largest crowds ... but he had the second-largest crowds, I will say," Trump said. "But I think you'll see his crowds now really start to dwindle. Because it's over."

Trump typically directs his attacks at Clinton, whom many consider the most likely Democratic presidential nominee, rather than Sanders. But over the last two days, he has started to dismiss Sanders and preview a potential general-election matchup against Clinton.

"I haven't even started on her yet," Trump proclaimed at a Sunday rally in Madison, Alabama.