One of my favorite sources is Nate Silver's blog FiveThirtyEight, even though he's been absorbed into the currently undistinguished national newspaper of record.
The question arises: why 538?
Because that is the number of electors in the Electoral College. OK. And why is that not equal to the number of Congress critters (435) + Senators (100)? It isn't because of the non-voting delegates (American Samoa, the District of Columbia, the Virgin Islands, Guam, and the Northern Mariana Islands) and one Commissioner (Puerto Rico).
It's because the 23rd Amendment to the U.S. Constitution says:
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