Here is the 23rd Amendment to the Constitution of the United States:
Passed by Congress June 16, 1960. Ratified March 29, 1961.
A number of electors of President and Vice President equal to the whole number of Senators and Representatives in Congress to which the District would be entitled if it were a State, but in no event more than the least populous State; they shall be in addition to those appointed by the States, but they shall be considered, for the purposes of the election of President and Vice President, to be electors appointed by a State; and they shall meet in the District and perform such duties as provided by the twelfth article of amendment.
Until the 1964 election, none of the residents of Washington DC had a say in who was to be President. They were citizens of the United States and many were not federal employees, but they had no input into the Presidency.
This was partially remedied in with this Amendment. I say partially because while they were given the right to have electoral votes, they were limited to the number available to the least populous state. Currently this means they have to have the same votes as Wyoming. Currently this doesn’t seem to be a problem as the District of Columbia ranked just above Wyoming and below Vermont in the 2000 Census, the last time electoral votes were apportioned. But it could become less fair in the future.
United States — States; and Puerto Rico
GCT-PH1-R. Population, Housing Units, Area, and Density (geographies ranked by total population): 2000
Data Set: Census 2000 Summary File 1 (SF 1) 100-Percent Data
http://tinyurl.com/2000compareStates (Original URL took up several lines of text)