The first presidential debate between Hillary Clinton and Donald Trump is in just over a month, and yet neither candidate knows who the moderator will be.

They won't know until after Labor Day, sources with knowledge of the matter told CNNMoney. The Commission on Presidential Debates had planned to announce moderators in late August, but it now intends to wait until after September 5, the sources said.

The delay is due in part to an unprecedented challenge the bipartisan Commission faces in selecting individuals who are immune (or at least as immune as possible) to accusations of bias. While that is always a concern for the Commission, the sources said it is more challenging than ever this time around due to one factor: Donald Trump.

Related: Donald Trump and RNC attack presidential debate schedule

The last thing the Commission wants is for the moderator to become part of the story about a debate. Yet Trump's aggressive attacks on the media and complaints about unfair treatment have effectively guaranteed that the moderators will come under scrutiny from conservatives.

This has made the Commission even more cautious than usual in researching potential moderators, sources said. The Commission fears that Trump would use even the slightest whiff of a pro-Clinton bias to attack a moderator and undermine his or her credibility.

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