Six hundred thousand American citizens living in Washington DC have no representation in Congress, not a vote in the Senate or the House of Representatives. Nevertheless, Congress has exclusive legislative authority over DC. Among other things, Congress reviews and modifies DC's local budget, and can annul any law it does not agree with it. That's good old-fashioned taxation without representation.
The DC Voting Act, a bipartisan bill, which would have granted DC one voting member in the House, was defeated last September after a minority of Senators, led my Minority Leader Mitch McConnell (R-KY), were able to maintain a fillibuster.
So, what does anyone have against a DC vote? The White House opposes it because the Constitution says only the states will be represented in the House, and DC is not a state. Opponents believe that DC's congressional delegate, who votes in committee but not with the whole House, is sufficient. There are also political motivations, to find out, read more. DC's representation would most likely immediately translate into one more representative for the Democrats, and more if the Senate is included. The DC Voting Act tried to appease this concern, by also giving an extra rep to Utah, a red state set to get an additional seat soon.
Constitutional interpretations notwithstanding, it is troublesome that the federal government has such geographical and legal influence over DC, while citizens are being denied democratic participation. Even the UN Human Rights Committee has called for full representation for DC residents.