[Gianni] Missed that word the first time around. Dissent is not illegal.
It's interesting that our friend capitan_refugio sets the trigger so high, when Lincoln himself seems to have set it a lot lower: sitting on a court case Lincoln didn't want decided, for instance, or simply asking a lot of awkward questions (as Lincoln had himself done in his 1848 "Spot Resolution" speech in the House of Representatives) like Congressman Vallandigham did -- Rep. Vallandigham being otherwise a Union man, if not necessarily a vote for abolition.
It didn't take much, as you emphasize, in the way of illegality to get you put in a federal prison -- just a few tart editorials, by the testimony adduced here.