"Will impeachment hurt Democrats or Trump?" Niall Stanage asks at The Hill.
The eternal third party hope is that large numbers from among both notional "independents" and the less dedicated partisans on both sides of the duopoly will pronounce a pox on both houses.
Maybe they will in 2020, but if so I expect the symptom to be depressed turnout, not a strong third party performance.
The kind of independent or third party candidate who could and would provide a rallying point for those who can't stomach either the "don't even bother to hide it" banana republicanism that the GOP under Trump has completely given itself to, or the barmy tinfoil-hat McCarthyist "everyone we don't like is a Russian asset" mania coming from the Democratic side, would probably have had to start making his or her public moves by now to come anywhere close to matching the duopoly's vote totals next November.
That's if such a candidate exists, and I'm not certain one does -- especially if we exclude from consideration the rare sociopath who thinks the likelihood of the system's collapse justifies the risk of tearing into that system from the outside rather than taking the usual sociopathic route of working it from the inside.