HARRISBURG, Pa. — Republican presidential candidate Donald Trump on Thursday offered rare words of praise for one of his Democratic rivals: Sen. Bernie Sanders, who Trump said should keep attacking front-runner Hillary Clinton.

"He's been tough on her. In fact, I'd like him to keep going because the longer he goes the more I'm going to like it," Trump told thousands of supporters during a campaign rally here in Harrisburg.

Trump lauded Sanders for keeping pressure on Clinton and particularly singled out Sanders’s comments attacking Clinton’s judgment. He also credited Sanders with being the first to question Clinton's qualifications for office.

"So Bernie Sanders, not me, said she is not qualified. So now I’m going to say, 'She’s not qualified.' OK?" Trump said.

Sanders, who is considered by most strategists to have almost no chance of overcoming Clinton's delegate lead, has been criticized by Democratic party strategists for escalating his attacks on Clinton, potentially weakening her ahead of an already tough general election. Sanders and his campaign have remained steadfast in their belief that they are not yet out of the running.

“In many respects, she may have the experience to be president of the United States,” Sanders said on NBC’s “Meet the Press”earlier this month. “No one can argue that. But in terms of her judgment, something is clearly lacking.”

Both Clinton and Trump have homed in on one another in recent days in anticipation of a possible general election match-up. Trump has given Clinton the derogatory nickname “Crooked Hillary.” Meanwhile, Clinton has pounded Trump on what she calls divisive and bigoted rhetoric.

Trump himself admitted that Sanders likely will not win the Democratic nomination — and said that he would prefer to go up against Clinton.

"No, Bernie's over. I guess, right? It's over," Trump said. "And really, I want to run against Hillary. I don't want to run against Bernie. I want to run against crooked Hillary Clinton."