President Obama acknowledged in a new interview that Democratic front-runner Hillary Clinton had shown a degree of "carelessness" in managing her emails while heading the State Department.

"I continue to believe that she has not jeopardized America's national security," the president told Fox News Sunday in an interview. But, he added, "what I've also said is that -- and she has acknowledged -- that there's a carelessness, in terms of managing e-mails, that she has owned, and she recognizes."

The use of a private email server by the former secretary of state is currently under investigation by the FBI. Earlier this year, the State Department said it had found 22 emails on Clinton's homebrewed server that had to be upgraded to "Top Secret," the highest level of classification. Those top secret documents were withheld from being publicly released along with the rest of her emails.

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The president noted the differing types of classified information, drawing on his own experiences handling such documents.

"What I also know, because I handle a lot of classified information, is that there are -- there's classified, and then there's classified," Mr. Obama said. "There's stuff that is really top-secret, top-secret, and there's stuff that is being presented to the president or the secretary of state, that you might not want on the transom, or going out over the wire, but is basically stuff that you could get in open-source."

Of the FBI's probe into the possible mishandling of that information, the president told Fox News host Chris Wallace that he could "guarantee" that politics would not get in the way of a fair investigation.

"I do not talk to the Attorney General about pending investigations. I do not talk to FBI directors about pending investigations. We have a strict line, and always have maintained it," he said. "I guarantee that there is no political influence in any investigation conducted by the Justice Department, or the FBI, not just in this case, but in any case."

In his wide-ranging interview -- the first with Fox News' political Sunday talk show since entering the Oval Office -- Mr. Obama also weighed in on the ongoing fight over Judge Merrick Garland, his Supreme Court nominee.

"Originally, the Republicans said they wouldn't meet with him at all," the president said, referring to a number of top Senate Republicans that had vowed not to hold hearings for Garland. "Now a number of them have already had meetings. And the questioning that they're having privately with Judge Garland, is something that should be done publicly, through a hearings process, so the American people can make their own assessment. But I recognize there's pressure on the other side. Our goal is just to make sure that the Senate does its job and treats him fairly."

He cautioned that if Republicans continue with their obstruction, it would set a precedent for any future Democratic majorities in Congress to do the same with any GOP nominations.

"They'll say, 'Let's wait for four years, and we'll take our chances on the next president,'" Mr. Obama said.

He added that he would continue to stand behind Garland through the rest of his term, even after November's presidential elections.

Mr. Obama also touched on the ongoing fight against extremist groups abroad, addressing criticisms that he has been too cavalier in his responses to terror attacks.

"There isn't a president who's taken more terrorists off the field than me, over the last seven-and-a-half years," Mr. Obama said. "I'm the guy who calls the families, or meets with them, or hugs them, or tries to comfort a mom, or a dad, or a husband, or a kid, after a terrorist attack. So let's be very clear about how much I prioritize this: this is my number one job."

Of his actions following recent major terror attacks, the president said that his responses have been appropriate.

"In the wake of terrorist attacks, it has been my view consistently -- that the job of the terrorists, in their minds, is to induce panic, induce fear, get societies to change who they are," Mr. Obama said. "And what I've tried to communicate is, 'You can't change us. You can kill some of us, but we will hunt you down, and we will get you.' And in the meantime, just as we did in Boston, after the marathon bombing, we're going to go to a ballgame."

Later, the president went on to answer a lightning round of questions, including an overview of his years in the White House.

His best day in office? "The day that we passed the healthcare reform."

His worst? "The day we traveled up to Newtown, after Sandy Hook."

The president also named his biggest accomplishment as "saving the economy from a great depression" and his worst mistake as "probably failing to plan for, the day after, when I think was the right thing to do, in intervening in Libya."