I think a 50 minute connection between AC and US is going to be difficult. As was pointed out before AC is now next to DL's gates. Thus, you are going to need to reclear security. Depending on which US gate you end up flying out if, you could easily have a 10 minute walk. Both of US and UA's piers at DCA are the worst in the airport in regard to security waits. On the plus side, security is usually pretty good at allowing people who are running late to cut the line. Be assertive! (Not too assertive, though!).

Long story short, if everything goes perfect, your flight is perfectly on time, you have a short walk to the US pier, and security is not too bad, you will make the flight. If anything is slightly off there is no way you will make that connection.