There is indeed a Justice Department policy against taking significant prosecutorial actions in the weeks leading up to an election if those actions might influence the elections outcome. But this is not a formal rule written down anywhere , nor has it been mandated by a court or by Congress. Its variously referred to as a norm, tradition or custom within the department. And theres not always agreement about what exactly the policy means or when it should apply.
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First, the policy is most critical when the Justice Department action directly relates to someone who is a candidate in the upcoming election. Thats not the case here: The president is not on the ballot, nor are any of the other people who could potentially be in Muellers crosshairs.
The concerns underlying the policy also are heightened when the Justice Department actions would be a surprise, an unexpected bombshell that reshapes the political landscape. But the details of Muellers investigation are widely known, and any potential implications are probably baked into the election already. Short of indicting the president himself, its hard to imagine much Mueller could do that would dramatically alter the current picture.
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Finally, the concerns about Justice Department action should be most pronounced when there is a risk the prosecution could seem politically motivated; if a prosecutor from one party could appear to be trying to take down a politician from the other. But despite the presidents repeated tweets about angry Democrats investigating him, Mueller is a Republican, and any steps he takes will be approved by Trumps own Republican deputy attorney general.
https://www.washingtonpost.com/opinions/dont-be-so-sure-mueller-will-back-off-before-midterms/2018/08/31/27cc802e-ad4d-11e8-a8d7-0f63ab8b1370_story.html?utm_term=.d4b66c9308d7
Realistically, Mueller probably will follow the policy and lay low. But don't buy the Trump/Giuliani line that he is required to or that there's a deadline.