First, it's clear that if nonstop is your preference, it's basically united.

If you are willing to give up non stop, I believe the choices turn into American (thru chicago), Northwest (which flies some nonstops from minneapolis to dca) and one screwy one -- Alaska Air has one of the few long nonstops to dca (from seattle). If you work close to downtown and can get out of work early, alaska has a 5:30 p.m. dca flight, that goes thru seattle, but would allow you to get back home on a friday night and you can catch the metro to dca.

fyi, in checking fares for my brother who lives in pdx, I've sometimes found some very screwy fares, where booking portland flights on american or united, is far more expensive than booking the flights on their partner airlines Alaska Air (American) or US Airways (united) though you fly on American or United.

as per the discussion of bwi/dca/iad, I personally hate connecting thru chicago (though it's where I'm from. Way too many delays). I vote for IAD over BWI, basically because from downtown, transit is more predicatable.

Driving to BWI is a mess and while getting to bwi on the train is fairly easy weeknights after work, you can have to wait awhile coming back on weekends. But basically even on weeknights, it's metro to amtrak to a bus to bwi, which is three opportunities to miss connections. Dulles is metro to a bus (which leaves every half hour from west falls church). You can have to wait but it's one connection.