In the Electoral College, that is. It's Democratic, but not THAT Democratic.
Washington D.C. has several characteristics that make it lean pretty far to the left - in particular, it's a dense urban center, and it has a very racially diverse population (as of 2020, it was 41% Black, 40% white, 11% Hispanic/Latino, and 5% Asian). For a Republican to win the popular vote in the district, the party coalitions would need to flip entirely, with Republicans becoming the party of urban areas and winning a significant portion of the Black vote. I can't see that happening.
There's also historical precedent; no Democratic presidential candidate has ever won less than 74% of the vote in D.C., and no Democratic presidential candidate has ever won the district by less than 55 percentage points. That margin has been increasing over time, too - no Democratic candidate has gotten less than 90% of the vote in DC since 2008, and Hilary Clinton's 86.77 percentage point margin in 2016 is the highest in the district's history (Biden comes second at 86.75).
I don't see this happening without a rogue elector, and in a district that's roughly 5% Republican, I have no idea where you'd even find a rogue elector.
