Bad press. Billy Bush, etc. and anything they could get.
-- Seems to me they would need paperwork with some patina of legality in order to take him out either before or after election. --
They don't need any "paperwork with a patina of legality" if he loses. The need for paperwork corresponds with asserting a finding that the government saw "collusion with Russia." The narrative that the Russians hacked the DNC on Trump's behalf is supposedly supported with non-government "Crowdstrike" material.
The value of having "government papers" comes into play only if he is elected, and then it is used to dodge criminal charges and to advance to impeachment and removal.
-- Just making up smack and announcing it via their handmaiden MSM/DSM would not be enough even before election, IMHO. --
It usually is enough. Most candidates would have folded early. One mistake they made was thinking Trump would be a weak opponent, a joke of a candidate. Otherwise he would have been attacked harder during the primary.
-- What are National Security Letters? --
National security letter - Wikipedia
National Security Letters | Electronic Frontier Foundation
National Security Letters: FAQ | Electronic Frontier Foundation <- quick read
National Security Letter Statutes <- dense/legalese
Very contentious surveillance tool. No warrant, no suspicion needed.
Thank you, C. I — and I am sure many others here — learn so much from you.