Washington, DC is a one party city as I would call it, because the city of Washington is exactly that. Not a state at all the, but the United States Capital the Federal City and they are essentially a one-party city. Like Chicago or Boston and other big cities in the country, where the Republican party let alone third-parties tend to struggle. It doesn't necessarily have to be because there are Washingtonians who tend to believe in economic freedom and like economic conservatism.
But Republicans haven't figured that out yet and haven't run Republicans like Rudy Giulani there. Who tend to be economically and fiscally conservative, but moderate-liberal on social issues. And the GOP isn't really much of a party at all to begin with in Washington. So Washingtonians tend to register Democratic so they can have a vote in the Democratic primaries. And a say in whose going to serve as the Mayor and serve on the City Council.