Donald Trump. Sara D. Davis/Getty Images Real-estate mogul Donald Trump said Tuesday that Sen. Bernie Sanders' (I-Vermont) presidential campaign is already "finished."

Trump, a leading Republican candidate, dismissed Sanders' bid for the Democratic nomination during a press conference held at Trump Tower.

He blasted Sanders for what many people saw as his standout moment in the first Democratic debate: defending former Secretary of State Hillary Clinton.

"When the email problem came up, Bernie Sanders lost his whole campaign. I mean, what he did was so stupid from his standpoint," Trump said.

Clinton, the Democratic front-runner, has for months battled negative headlines about her exclusive use of a private email server and account for her government work at the State Department. Republican candidates regularly hammer her for the controversy, and the FBI is looking into whether any classified information was mishandled.

But when the topic came up at the CNN debate last month, Sanders declared "the American people are sick and tired of hearing about your damn emails!" Sanders' quip was one of the most memorable lines of the debate, and it was even featured in a "Saturday Night Live" sketch.

Trump clearly felt that it was a mistake, however, and he said Tuesday that unless the email controversy spirals out of control, Clinton will defeat Sanders in their primary contest.

"In order to get a one-minute sound bite and some applause, he gave up the emails," the businessman lamented. "That was the end of his campaign. First of all, people aren't going to his rallies anymore. "He's finished. Unless something happens to her with respect to the emails, she'll easily be the candidate."

Sanders responded later in the day by pointing to his general-election polling numbers against Trump: