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 Polymarket assigns to that outcome at midnight ET before election day. This will be measured by the state of the Automated Market Maker.

This market will resolve PROB to (1 + Y - X) / 2. If Polymarket is an unbiased estimator, 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.