Downtown Washington
-------------------------------- 2020 ------ 2010 ------ 2000 ------ 1990 ------ Growth ------ Density
Downtown --------------------- 30,279 ----- 27,025 ----- 18,704 ----- 17,457 ---- 12.0% ---- 44.5% ---- 7.1% ------ 11.0 km² --- 2,743.7 inh./km²
Georgetown -------------------- 13,603 ----- 14,231 ----- 12,991 ----- 12,181 ---- -4.4% ----- 9.5% ---- 6.6% ------- 3.2 km² --- 4,268.3 inh./km²
Washington DC --------------- 689,545 ---- 601,723 ---- 572,059 ---- 606,900 ---- 14.6% ----- 5.2% --- -5.7% ----- 158.3 km² --- 4,355.9 inh./km²
Arlington-Alexandria-F.C. ---- 412,768 ---- 359,925 ---- 328,113 ---- 291,697 ---- 14.7% ----- 9.7% --- 12.5% ----- 111.3 km² --- 3,708.3 inh./km²
Washington Metro Area ----- 6,105,431 -- 5,388,326 -- 4,635,194 -- 3,997,373 ---- 13.3% ---- 16.2% --- 16.0% -- 12,403 km²
Washington Downtown is very different from everything. Firstly, the Mall and Potomac Park takes half of it (1 census tract). The other 10 census tracts comprise Foggy Bottom and everything between the Massachusetts Avenue and the Mall. As this region is full of big government offices, embassies, hotels, it's hard to make any assumption about its residential population trends.
As bonus, I brought Georgetown, formed by 4 census tracts.