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Speaking to a crowd in Albany, New York, the Republican presidential frontrunner echoed Sanders' assertion that "something is clearly lacking" in Clinton's judgment.
"Now, he's saying bad things about Hillary. And he's really correct. He says she doesn't have the judgment to be president," Trump said, referring to Sanders.
Trump specifically criticized Clinton's handling of the military intervention in Libya while serving as secretary of state. He also bashed her initial support for the 2003 invasion of Iraq, though Trump himself has a mixed record on the issue.
"The one thing I agree, he's absolutely right: Hillary Clinton doesn't have the judgment to be president," Trump said.
Trump's Monday statement was an extension of his commentary on the Democratic primary the day before.
"You saw what Bernie said? They're starting to get angry at each other," Trump told a campaign rally audience on Sunday. "Bernie's saying she shouldn't be able to be in that position, because she's made so many bad decisions - she shouldn't even be able to be running for the position."
"And she has made bad, bad decisions. I look so forward to going after Hillary."
In interviews over the weekend, Sanders repeatedly said that while he believed Clinton is qualified to be president, her initial support for the invasion of Iraq and her acceptance of special-interest money cast a negative light on her judgment.
The Clinton and Sanders campaign have been fiercely sparring over the judgment issue in recent days.