Then you run into the problem of "What is authoritative?" By limiting it to just two, Twitter can say, "We like these guys. They're reputable." Instead of having to line up and vet thousands of other sources, and task people with monitoring all those thousands of authoritative sources around the world.

I’ve never understood the dewy beats Truman calling system. In the UK each constituency announces results after being counted (and recounted). Those results arrive during the night, polls close at 10, first results at 11, a good idea of who will win in most elections by 5, andthe last few results might stretch into 9 or 10 am if ferries are having issues.

But aside from a national exit poll at 10pm, there are no real guesses.

It seems that different US elections predict who will win a given state far earlier in the process - before the votes are counted even the first time. Is that right?

My own global broadcaster takes its “calls” from ABC. I think that abc, nbc, cbs, Fox, cnn, New York Times and Washington post are probably good enough for “calling it”. I was working election night 2012 in our Washington office and was shocked how early the “Obama wins” straps went up -I have a photo at just 23:19 eastern from fox saying Obama was re-elected. That’s something like 4 hours before polls close in Alaska and Hawaii!

If 4 of those networks and papers have called it then it’s likely good. Sure there’s buzzfeed and whatever that may have enough professional staff on to really judge it, but it’s unlikely that 3 networks and a major paper are all going to call Dewy rather than Truman.

> It seems that different US elections predict who will win a given state far earlier in the process - before the votes are counted even the first time. Is that right?

Yes.

At least in Dewey / Truman, they counted the votes and Truman won. Under the current system, Truman would be expected to concede gracefully after somebody predicted he would lose, removing the need to count the votes entirely.