Let X be 1 if a Republican wins the next US presidential election, 0 otherwise. This will resolve the same as the following market on Polymarket: https://polymarket.com/event/which-party-will-win-the-2024-united-states-presidential-election

Let Y be the probability FiveThirtyEight assigns to that outcome at midnight ET before election day. If they have several models, this will resolve as per the most prominently displayed on their homepage. If that is not unique either, this takes the model incorporating the most information.

This market will resolve PROB to (1 + Y - X) / 2. If Polymarket is an unbiased estimator (i.e. X and Y have the same expected value), then the expected value of this is 50%. If Polymarket is biased in favor of Republicans, then the expected value of this would be larger.