And the party's over...and it's time to get over my awesome hangover...uh, I mean, I'm having a headache right now...heh heh...
I doubt enfranchisement would have worked, or even been thought of. Today DC is a prominent second-rank city in the US, but back then was another story. Living conditions were sufficiently unpleasant that many European diplomats considered it a hardship posting and many politicians, jokingly or otherwise, wanted to move it entirely. I would personally doubt the city itself, or even the entire district, had sufficient population to claim a single member of the House of Representatives. Further, back then, the only basis for the city's survival was the presence of the US government. Had the seat of government been moved it is not hard to foresee DC in the early 21st Century as a minor suburban area.
As late as the 1930s it qualified as a minor, if more pleasant, southern city.
More importantly, the original founding of DC came from land provided by Maryland and Virginia, and when the area from Virginia was found to be surplus, it was simply returned. No debate over a new state, or who would get it. It left Virginia solely to be part of the federal district and had no legal basis outside Virginia otherwise. It is certain that in the 19th Century the same would have applied to the rest, vis a vis, Maryland. In fact, a strong argument could be made today to this effect.
Sean, I don't see how expanding would be useful once the war was won, neither does the likelihood of a peace-time fortification program seem likely. If people think the Brits were quick to gut the military once peace broke out, they should see how the US went for the 'peace dividend' back then.
It's hard to see where the changes would take place, presuming an early loss of the capitol in 1861. I really can't see an instant expansion of the military or the capacity to field armies. Allan Nevins, one of my favorite historians on the subject, examined this in some detail and considers it a major success for the Union to hold what they did in 1861. Also, given that the hard-liners were the ones calling for the advance which had now blown up so badly, it might be that the more pragmatic elements of the GOP might have been encouraged.
We should bear in mind that this situation presumes the CSA managed to combine two smaller armies for the Battle of First Bull's Run, and used them even more effectively(I don't see where more armies would have come from) to crush the Union forces. This leaves the Confederates tired and bloodied, and the second Union force unscathed, not to mention any Union forces in DC itself or on the south bank of the river. Yes, let's not forget that there was a great river between the CSA and DC, not to mention some kind of Union flotilla. I think we need more info to start a CSA assault on DC at this time.