Sen. Bernie SandersBernie SandersButtigieg on Biden's Iraq War vote: 'that vote was a mistake' Buttigieg on Biden's Iraq War vote: 'that vote was a mistake' The generational divide of Joe Biden and the Democratic Party MORE (I-Vt.) blasted Donald TrumpDonald John TrumpTrump defends Stephanopolous interview Trump defends Stephanopolous interview Buttigieg on offers of foreign intel: 'Just call the FBI' MORE as the “least qualified person to become president” immediately after the GOP nominee criticized him during Wednesday's presidential debate.

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At this year's final debate, in Las Vegas, Trump referenced one of Sanders's attack lines from the Democratic presidential primaries — that Hillary ClintonHillary Diane Rodham ClintonHouse Intel Republican: 'Foolish' not to take info on opponent from foreign ally House Intel Republican: 'Foolish' not to take info on opponent from foreign ally It's about the delegates, stupid MORE has bad judgment — and used the current battle to retake Mosul from the Islamic State in Iraq and Syria (ISIS) as an example of this.

"Bernie Sanders he said you have bad judgment. You do," Trump said. "And if you think that going into Mosul, after we let the world know we were going in, and all of the people that we really wanted, the leaders, they’re all gone. If you think that was good, then you do."

The former secretary of State pushed back on Trump's criticism, noting that she's earned her primary rival's endorsement, which Trump added was "a big mistake."

“Well you should ask Bernie Sanders who he’s supporting for president, and he has said as he has campaigned for me around the country you are the most dangerous person to run for president in the modern history of America. I think he’s right," the Democratic nominee said.

Sanders swiftly took to Twitter after the heated exchange to back up Clinton's argument.

“Donald Trump is the least qualified person to become president in the history of our country. Go vote!” Sanders tweeted after Trump's jab.