White House press secretary Josh Earnest on Thursday referred to the probe into Hillary Clinton's private email system as a possible "criminal investigation," which Republicans quickly seized on as a critical slip just moments after President Obama endorsed Clinton for president.

Earnest was asked whether the FBI might be pressured to go easy on Clinton in the investigation in light of Obama's endorsement, and used the term "criminal investigation" in his answer.

"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," he said.

Clinton has tried to downplay the investigation as a "security review," but Republicans jumped at Earnest's formulation as a sign it may be much worst for Clinton. The Republican National Committee sent out an email arguing that Earnest "lets slip that the FBI probe into her emails is criminal in nature."

"The White House's admission that the FBI is investigating Hillary Clinton's email server as a 'criminal' matter shreds her dishonest claim that it is a routine 'security inquiry,'" RNC spokesman Michael Short said in a statement Thursday afternoon. "This is another reminder of her reckless conduct as Obama's secretary of state, where her attempt to skirt government transparency laws exposed highly classified information and put our national security at risk."

Earnest also rejected the idea that the FBI would be affected by Obama's endorsement.

"And you have noted a couple of instances in which the president had been asked about the FBI investigation, and in each of those answers, the president made clear that that investigation is one that is being conducted independent of any political interference — that is a principle to which the president is," Earnest said.

Earnest added that the reason Obama believes he can "go out" and make a strong endorsement and record video pushing for her election is that he knows that the Justice Department investigators involved in the Clinton probe "aren't going to be swayed by any sort of political interference."

"They aren't going to be swayed by political forces," he said. "They know that their investigation should be guided by the facts and that they should follow the evidence where it leads … the president has complete confidence that's what they'll do."

Earnest said Obama has never discussed the Justice Department's email probe with Clinton. Obama was set to meet privately with Attorney General Loretta Lynch later Thursday, although the White House has not said what they plan to discuss, and Obama routinely meets with his cabinet officials on a regular basis.