Don't put blind faith in the airlines to get you there on time, they have the power to massively stifle the number of people attending the protest in DC.

This is far too important to not have time needed or a backup plan if the flights are completely full or the airlines decide to mass cancel flights the day before

Unlikely? Maybe. But so is a blatantly fraudulent election shamelessly covered up.

I think pedes should consider flying into the surrounding areas. That way, in the worst case scenario you can haul ass in other ways to get there.

As per u/Albgardis:

You can fly into Philly and take a megabus for $5-10 (price depends on daytime and date) into DC.

Or fly to JFK, EWR or LGA and take Amtrak from NYP station in Manhattan.

I am in PA and familiar with connections like this. Ask me if you need advice.