The short answer is democracy, but there are two specific aspects:
1. As a state, DC residents would get their representation in the House and the Senate. Right now they get only non-voting members.
2. The Senate is a undemocratic institution and its malapportionment is giving a huge unfair advantage to Republicans. The Senate is currently split 50/50 but those 50 Democratic Senators received many millions more votes than their Republican counterparts and represent tens of millions more citizens. DC is very blue and would reliably send two Democrats to the Senate. It doesn’t fix the problem, but the practical result is to make an unrepresentative institution slightly more representative.
There are a lot of reasons. Just a few:
* Taxation without representation–DC residents pay taxes but have no representatives in the legislature to speak for them
* Autonomy–they can make their own laws governing themselves
* In the 1/6 attempted coup, no one in DC government could call in the national guard, they had to wait for the president or secretary of defense to do so. Since those guys wouldn’t do it, there was a wait of hours while the mayor of DC and the governor of Maryland desperately tried to get someone to do it. If DC were a state, the governor of DC could call in the national guard
* population: DC has a population larger than that of a couple of states (Wyoming and Vermont) yet they have no representation
* adds democrats to congress. If it becomes a state, DC will likely add 2 democratic senators to congress and 1 democratic congressman to the House. So for that reason, Republicans are pretty opposed to DC statehood while democrats tend to favor it.
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