Interestingly, Washington DC has voted Democrat every year since 1964 when it was added to the election. The closest a Republican has got was 1972 when Nixon lost by "only" 56.5%
The reason why Minnesota has voted for Democrats longer than all other states is because they were the only state (DC doesn’t count) won by Walter Mondale in 1984, being his home state and even then, barely winning, with a margin of 3761 votes. This gave Reagan 525 electoral votes out of 538.
The District of Columbia is part of the Electoral College and should be part of this quiz. A line in the caveats should be sufficient to clear up the slight irregularity around the word "state."
They have been Democrat since 1992.
https://en.wikipedia.org/wiki/1988_United_States_presidential_election