You never did explain why you think it's so crazy? What makes it objectively wrong to enfranchise people, rather than the subjective issue that it'll help the "other side"? If DC would vote solidly Republican would it still be bad?
It would violate the constitution, for one.
For another, DC already has far more influence on the Federal government than any state does, by simple fact of the local culture dominating so much of the apparatus of the federal bureaucracy. There are good reasons for the administrative district of a nation to have sharp limits on what influence it holds in balance to the rest of the nation. Part of that is because it gives the administrative state an additional level of sway over whether or not to increase their own authority and funding, something they already have by far altogether too much of.
On top of that, being a resident of DC is strictly voluntary. Anyone who lives there can more. It takes ~6 months to a year to establish residency within a state, and you aren't required to
change your registry even if you work or live elsewhere most of the year. Beyond
that, DC residents get to elect their city government.
If you are seriously worried about enfrachisement, there's a
much simpler solution than making an area less than 70 square miles (stronger than pretty much all
Counties) into a state. Just create a legal setup where everyone who lives in DC is still legally a resident of a different state. This would fit well for elected officials in a symbolic way as well. Then you can vote with the rest of your state, and there's a much-reduced conflict of interest.
No, the real reason the Democrats are pushing this is because DC is deeper blue than California or any blue state, and as more and more power accrues to the federal bureaucracies, this trend will only intensify. It would take a shocking national event on the level of a world war to have a
chance at changing the entrenched local culture, and so it's literally just an attempt to gain permanent Democrat seats in the Senate and House.