Donald Trump has secured enough delegates at the Republican convention in July to become the Party's Presidential candidate.
That means the billionaire property developer is now officially unstoppable in his quest to lead the Party into the November election.
Meanwhile, President Obama, speaking in Japan, said world leaders are telling him they are increasingly worried about the possibility that Donald Trump could be America's next President.
Our Washington Correspondent Simon Marks reports.
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There is a long tradition in the United States that when a President travels overseas, his political opponents back at home do not criticize what he's doing while he's on-the-road.
That would be seen as undercutting the head of state while he's engaged in stately business.
But it does not apply the other way around, as Barack Obama made very apparent in remarks to reporters in Tokyo about the man who wants to succeed him.
Donald Trump was on the President's lips….because, he said, the possibility of a Trump Presidency is rattling the very world leaders that he'd been meeting at the G7 Summit.
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And lest anyone is in the dark about the reasons why people should be rattled over Mr. Trump's sudden rise….the President elucidated some of them.
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President Obama's comments came just hours before Donald Trump learned that he is now officially unstoppable in his quest for the Republican Party's Presidential nomination.
He needed 1,237 delegates at the party's convention in July to secure victory…..he's now surpassed that number.
It is an astonishing achievement for a man who was initially derided and written off when he first entered the Presidential race late last year.
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Mr. Trump is bathing in the glory in North Dakota where he's making campaign appearances.
And hitting back at Barack Obama….saying that if world leaders are troubled by the Trump train….that's just another reason to vote for him.
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He's also tweaking Hillary Clinton, who still hasn't sewn up the Democratic Party's nomination because of the opposition she continues to face from socialist senator Bernie Sanders.
She's also dogged by continuing questions and criminal probes into her use of a private e-mail account while she was Secretary of State in violation of the department's rules.
Donald Trump is clearly among those who find it extraordinary that he's won the Republican nomination before she's sealed the Democratic Party's deal.
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Back in Tokyo, President Obama was asked about the increasingly angry disarray within his own party.
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In private, the President is believed to be chomping at the bit for the Democratic nomination battle to be over so Hillary Clinton can begin to try and claw back the lead that she once enjoyed in the polls over Donald Trump.
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