WASHINGTON—In a searing denouncement, U.S. 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.

A chorus of Republicans has 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.

Instead, it was Trump stunningly withholding his support from top GOP lawmakers, including House Speaker Paul Ryan.

In an affront to his party’s top elected official, Trump told the Washington Post he wasn’t “quite there yet” on an endorsement for Ryan in his primary next week.

Trump’s refusal to back Ryan exposed anew the deep divisions within the GOP and underscored that the businessman rarely plays by the traditional political playbook.

Ryan has been among those urging Republicans to rally around Trump, despite concerns about his candidacy.

Ryan’s campaign said, “Neither Speaker Ryan nor anyone on his team has ever asked for Donald Trump’s endorsement. And we are confident in a victory next week regardless.”

Trump also said he was not supporting Sen. John McCain in his primary in Arizona, and he dismissed Sen. Kelly Ayotte as a weak and disloyal leader in New Hampshire.

So far, one elected federal Republican, Richard Hanna, has declared he’ll be voting Democrat.

Hanna is uniquely placed to do so. He is a popular third-term congressman who won his last election by almost 50 per cent; he’s in a swing district in New York that split its votes in the last presidential election; and, perhaps most importantly, he’s not running again.

Meanwhile, a prominent Republican fundraiser, Meg Whitman, said Tuesday she would support the Democratic nominee, Hillary Clinton, for president and give a “substantial” contribution to her campaign in order to stop Trump, whom she berated as a threat to American democracy.

“I will vote for Hillary, I will talk to my Republican friends about helping her, and I will donate to her campaign and try to raise money for her,” Whitman said in a telephone interview.

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She revealed that Clinton had reached out to her in a phone call about a month ago, one of the first indications that Clinton was aggressively courting Republican leaders.

Whitman, a Hewlett Packard Enterprise executive, acknowledged she diverged from Clinton on many issues, but said it was time for Republicans “to put country first before party.”

With files from Star wire services

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