President Obama will go down as perhaps the worst president in the history of the United States! — Donald J. Trump (@realDonaldTrump) August 2, 2016

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Republican presidential nominee Donald Trump lashed out at outgoing US President Barack Obama after the latter issued one of his harshest criticisms of the billionaire real-estate mogul, calling him "unfit" to serve as president."President Obama will go down as perhaps the worst president in the history of the United States!" Trump tweeted on Tuesday night.Trump's retort came after Obama denounced him as being “woefully unprepared” to serve as president of the United States.Speaking Tuesday at a joint news conference with Singapore’s prime minister, Obama said that Trump lacks the judgment, temperament or basic understanding of the world around him necessary to lead the world’s most powerful nation.“I think the Republican nominee is unfit to serve as president,” Obama said. “I said so last week. He keeps on proving it.”In addition, he called on Republicans to condemn and repudiate their party’s nominee.Obama spoke of Trump’s nomination, which the candidate formally accepted two weeks ago in Ohio, several days into a controversy involving the GOP candidate’s attacks on the Muslim family of an US Army officer, Capt. Humayun Khan, who died heroically in Iraq.Trump’s decision to criticize a Gold Star family has invoked a deluge of condemnation from Republicans and Democrats alike. But members of the GOP who have already endorsed him refrained from fully revoking their support.“What’s been interesting has been the repeated denunciations of his statements by leading Republicans, including the speaker of the House, the Senate majority leader, prominent Republicans like [Sen.] John McCain,” Obama said.Michael Wilner contributed to this report.