President Obama gave a ringing endorsement of presumptive Democratic Party nominee Hillary Clinton Thursday – even saying he doesn't believe "there's ever been someone so qualified to hold this office" – despite the White House simultaneously admitting for the first time that Clinton is under "criminal investigation" by the FBI.

After news of the endorsement broke, a Twitter war broke out between Clinton and GOP presumptive nominee Donald Trump.

Upon hearing the news, Trump tweeted, "Obama just endorsed Crooked Hillary. He wants four more years of Obama – but nobody else does!"

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That's when Clinton snapped back with a demand that Trump delete his Twitter account.

Trump returned fire: "How long did it take your staff of 823 people to think that up – and where are your 33,000 emails that you deleted?"

Even Republican National Committee Chairman Reince Priebus jumped in on the action with his own tweet: "@HillaryClinton If anyone knows how to use the delete key, it's you."

Watch President Obama's endorsement of Hillary Clinton:

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In a video of his endorsement, Obama said:

"For more than a year now, across thousands of miles, and all 50 states, tens of millions of Americans have made their voices heard. Today, I just want to add mine. I want to congratulate Hillary Clinton on making history as the presumptive Democratic nominee for president of the United States. Look, I know how hard this job can be. That's why I know Hillary will be so good at it. In fact, I don't think there's ever been someone so qualified to hold this office. "She's got the courage, the compassion and the heart to get the job done. And I say that as somebody who had to debate her more than 20 times. Even after our own hard-fought campaign, in a testament to her character, she agreed to serve our country as secretary of state. From the decision we made in the Situation Room to get [Osama] bin Laden to our pursuit of diplomacy in capitals around the world, I have seen her judgment. I have seen her toughness. I've seen her commitment to our values up close. I've seen her determination to give every American a fair shot at opportunity, no matter how tough the fight was. That's what has always driven her, and it still does. "So I want those of you who've been with me since from the beginning of this incredible journey to be the first to know that I'm with her. I am fired up, and I cannot wait to get out there and campaign for Hillary."

But there's one major problem with Obama's endorsement, according to Judge Andrew Napolitano.

The judge warned that Obama's endorsement presents a major conflict of interest while Clinton remains under criminal investigation for using an allegedly illegal private email system to conduct official government business as secretary of state.

"You're talking about hundreds of human beings, federal prosecutors, FBI agents, investigators, clerks, researchers, and thousands of person hours investigating Mrs. Clinton," Napolitano told Fox News Thursday. "These people are saying, well, our work could now be nullified because of the president's political wishes. He's not saying stop investigating her. He's not saying we're not going to present this to a grand jury. But how can we continue with this investigation knowing that our boss wants this person who is the target of the investigation to succeed him? That's called a conflict."

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In fact, it was Thursday that the White House publicly admitted that Clinton was the subject of a criminal investigation.

When White House Press Secretary Josh Earnest was asked about the "potential conflict of interest" of a sitting president endorsing a candidate who is being investigated by the FBI, he insisted the investigations are "being conducted without any political interference."

"I think those career prosecutors understand that they have a job to do. ... They don't have political jobs. They have career jobs as law enforcement officers and as prosecutors and investigators. That's what their responsibility is. And that's why the president, when discussing this issue in each stage, has reiterated his commitment to this principle that any criminal investigation should be conducted independent of any sort of political interference and that people should be treated the same way before the law regardless of their political influence, regardless of their political party, regardless of their political stature and regardless of what political figure has endorsed them" (emphasis added).

A reporter then asked, "Has President Obama ever discussed the Department of Justice investigation with Mrs. Clinton?"

Earnest replied, "He has not."

Watch a video of statements made by White House Press Secretary Josh Earnest:

Priebus responded to Obama's endorsement of Clinton, saying she "put our national security at risk" and set up her private email server to "obscure her unethical dealings."

"I don't think there's been a candidate more unethical and untrustworthy than Hillary Clinton," he said.

A full 60 percent of Americans believe Clinton is lying about her emails and another 57 percent say she put national security at risk, according to a new Fox News poll released Thursday. However, Clinton beats Trump in a head-to-head match-up 42-39 in the same poll.

In yet another possibly related development, Obama held a meeting with Attorney General Lynch at the White House just after his endorsement was made public. It was closed to the press.

Immediately after the endorsement, Clinton's campaign announced Obama would be campaigning with her at events in Wisconsin.

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