Republican presidential nominee Donald Trump said Tuesday that President Barack Obama was "the worst president, maybe in the history of our country" after Obama called Trump "unfit to serve" and "woefully unprepared to do this job" earlier in the day.

"I think he's been a disaster. He's been weak, he's been ineffective," Trump said on Fox News' "The O'Reilly Factor." "I believe I know far more about foreign policy than he knows. Look at Ukraine. He talks about Ukraine [and] how tough he is with Russia, in the meantime they took over Crimea."

Earlier Tuesday, Trump said that Obama and Democratic presidential nominee Hillary Clinton have “single-handedly destabilized the Middle East” while putting the “country at risk” with Clinton’s use of a private email server.

"She is reckless with her emails, reckless with regime change, and reckless with American lives,” Trump said.

The real estate mogul also restated his concern that the November election would be "rigged". He told host Bill O'Reilly that recent court rulings striking down voter ID laws in Wisconsin and North Carolina meant that people would "vote 10 times, maybe. Who knows?"

"I am very concerned and I hope the Republicans are going to be very watchful," Trump said.

Obama delivered his broadside against Trump while fielding a question at the top of a White House press conference with the visiting prime minister of Singapore. Obama diverted from the central topic of that visit – moving along the controversial Trans Pacific Partnership trade deal, which Trump opposes – to fundamentally question whether the federal government could function properly if Trump wins.

“I think the Republican nominee is unfit to serve as president,” Obama said, adding, “He keeps on proving it.”

The president questioned whether Trump has “basic knowledge” on key issues. He went on to say that with past Republican nominees – including his former rivals John McCain and Mitt Romney – he never had doubts about their ability to do the job of president even though they disagreed on policy.

“Had they won I would have been disappointed, but I would have said to all Americans, this is our president,” Obama said, noting he was confident they would abide by certain rules and observe “basic decency.”

Obama added: “But that’s not the situation here ... There has to come a point at which you say, 'Enough.'"

The Associated Press contributed to this report.

