"I think the Republican nominee is unfit to serve as president. I said so last week and he keeps on proving it," Obama said on Tuesday during a joint press conference with Singaporean Prime Minister Lee Hsien Loong at the White House.
Obama said Trump is "woefully underprepared" to be president. "He doesn't appear to have basic knowledge on critical issues in Europe, in the Middle East, in Asia," Obama said.
Obama said this opinion of Trump is shared by prominent Republicans.
"I think what's been interesting is the repeated denunciations of his statements by leading Republicans including the speaker of the House, the Senate majority leader and prominent Republicans like John McCain," he said. "The question I think that they have to ask themselves is, if you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?"
At some point, Republicans should realize that Trump has gone too far and that the things he says indicates he "doesn't have the judgment, the temperament the understanding to occupy the most powerful position in the world," Obama said.
While Obama said he has disagreed with former Republican presidents on policy issues, he "didn't have a doubt that they could function as president."
Obama explained that had he lost to Mitt Romney or McCain, he certainly "would have been disappointed," but that he would have conceded and had the confidence that they would "abide by certain norms and rules and common sense, will observe basic decency."
Later, Trump responded in a statement on Facebook, alleging that Obama and Democratic presidential nominee Hillary Clinton "have single-handedly destabilized the Middle East" and that "we need change now."