The metro could be shut down. Planes could be cancelled.
Driving is probably the best way to get there. I made a hotel reservation in D.C. before I realized I probably won’t fly. The best route is probably drive, have hotel in Virginia, march by foot en masse across the river to D.C.
I strongly suspect President Trump will not allow the Metro to be shut down.
DC is federal territory.
I disagree with coming thru VA, if they close the main access, you're down to Arlington Memorial or Key on foot.