I have flown to DCA numerous times on AS out of SEA, PDX and LAX. As an MVP I find the best chances of getting an upgrade is PDX-DCA followed by LAX-DCA. The chances of getting an MVP upgrade out of SEA is about zero.
If cheaper fare is your goal look at DL to BWI, almost always much cheaper than DCA. There is a direct bus link from BWI to the DC Metro system.