There isn't any direct bright line here, unfortunately. The "fire in a crowded theater" is the most apt from SCOTUS, but even that doesn't cover what Marge and their Terrorist Members are doing here.
But your question is germane. How the fuck does speech like that fly? Two reasons: we value speech above a LOT of other things, and were reluctant to squash speech without GOOD reasons. Obviously this is a good one, but it doesn't go so far as to literally say "go kill them son's of bitches" - she's just speaking euphamistically, and while it's Deplorable, what we dont' want to do as a rule of law is "guess" someone's intentions behind their speech, if that speech isn't clear - which in Marge's case it isn't "legally" clear, if that makes sense. Second, Marge is a Member of Congress, and they are afforded even more protection than a normal citizen, given the vary nature of their speech is political.
What's been really awful about the GQP Deplorables is that they are taking honorable things (like member of congress "protections", the White House, little things like that) and shitting all over them. Taking full advantage of protections and using them to push lies and conspiracies and treason and terrorism.
There have been several solid articles out there about how the GQP is using the "Gentleman's Agreement" rules of politics in our system and twisting them to keep a minority party ruling a majority. But that's another conversation.