If you showed at the duck hunt in Carhartt, you'd fit in -- in Washington D.C. not so much.
Suits are the uniform of the day there.
If you are concerned about roadblocks or attracting negative attention on the way in, considering traveling wearing a business suit.
If you are young you are a staffer. It doesn't have to be an expensive suit. You might even get away with a jacket, shirt, tie and khaki-type pants that don't match.
If you are older, you can still be a staffer (cheaper suit), or a lobbyist (expensive suit)
If you don't have a business haircut, you are an IT or other high-skill consultant that doesn't have to fully comply with norms.
If you have a military haircut and demeanor, you're consulting the Pentagon or are private security of some kind.
If there are 4 of you in a car, you are a consulting team. The older person is the team leader and is probably not driving.
White or Light Blue shirt. Tie probably should not be a color that screams politics.
Fit one of the models above, and chances are, if you get a look over, they'll think nothing of it and move on to someone who looks more "interesting".
Suits can suck if you are not used to them, but they invoke that Jedi Mind Trick: "These aren't the droids you are looking for."
Once on site, you can change out of the suit, or if you planned a personal visit to your Congresscritters to say thanks or something more pointed, that part of your visit should go easier.
As much as I would say visit your congressional office that day. On that day without force you aint getting in. Its called crowd control and Capital hill police, uniformed protection officers, USSS ,park polce are extremely good at it. Going in disguise is a good idea however there are too many ways to get in to DC so that could be moot. Skip the metro park in garages with others as close to Capital Hill and the white house as you can. Reagan national is not in DC its across the river in VA. Long walk real long. Get in Town early like days early. Seem some folks plan on camping sounds great and there are some kick ass ventilation vents that pump warm air outside but you'll have to battle homeless for those golden ticket spots. Best bet is to converge from Maryland side anywhere thru PG County. Maybe thru southeast DC its a little Ghetto but no worries plan your routes. Good luck and Gods Speed
Enough Patriots and all that "crowd control" jive is moot.