I did the tourist thing last August in DC. Most places, including Air & Space, were just ushering people right in with no bag check and no x-rays or magnetometer. One Smithsonian museum - Native American - did have some goofball at the door who eyeballed people, and he yelled at me to stop when he saw the top of my Streamlight ProTac 2AAA sticking out of my pocket, but when I showed him what it was he went back to being a door stop. The Holocaust Museum is serious about their [bleep], so don't try anything wacky there. I was denied admission to the Capital because of the Vic Classic SD in my pocket, so I hid it outside and retrieved it after I left. I knew it was off limits, but I tried it anyway. The Senate building was fine with my Schrade "tactical" pen, but they have real security there too.
A word to the wise - don't wear a Wilderness Instructors belt like I did. The dang buckle is metal, so you have to take it off to go through magnetometers like at the Capitol. So wear a belt with plastic buckle. Were I to go back to DC, I would put all of my goodies, especially my metal goodies and phone, in a backpack so you don't have to stand there like a goofball at a checkpoint and find your phone, your keys, your tactical pen, your whatever. Its all in the backpack and you just hand it to the guard.
So I guess it really depends on whether the Smithsonian museums are doing actual security checks, or just herding people right in the front door with no checks in order to keep the line moving like I experienced. Only bad people are allowed to have knives and guns in DC, so forget about defending yourself. Rock on.