In a searing denouncement, President Barack Obama castigated Donald Trump as “unfit” and “woefully unprepared” to serve in the White House.

He challenged Republicans to withdraw their support for their party’s nominee, declaring “There has to come a point at which you say enough.”

While Obama has long been critical of Trump, his blistering condemnation Tuesday was a notable escalation of his involvement in the presidential race. Obama questioned whether Trump would “observe basic decency” as president, argued he lacks elementary knowledge about domestic and international affairs and condemned his disparagement of an American Muslim couple whose son was killed while serving the U.S. Army in Iraq.

Several Republicans disavowed Trump’s criticism of Khizr and Ghazala Khan and the Republican nominee’s calls to temporarily ban Muslims from coming to the U.S. But Obama argued that isn’t enough.

“If you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?” Obama asked during a White House news conference.

“What does this say about your party that this is your standard-bearer?” No prominent Republican lawmaker responded to Obama’s challenge. Trump’s response? On Twitter, he said, “President Obama will go down as perhaps one of the worst president in the history of the United States!”

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