CommutAir might be an option for you, while not the greatest pay (apart form the highest first year pay) they historically have a short upgrade time.
Out of TPA you'd have 3 United flights/day to IAD,
or you could take one of the 9 flights to DCA (JetBlue, Southwest, US Airways), and then take the Metro to IAD.

They also have a commuter clause that's unheard of in the regional industry.