The electoral college bias is defined as the difference between the Democratic margin of victory[1] in the tipping-point state minus the Democratic margin of victory in the national popular vote, in the 2024 presidential election. The tipping point state is defined as, if you were to order all states from highest to lowest by Democratic margin of victory, the one that would get the Democratic candidate[2] to 270 electoral votes[3].

For example, in the 2012 presidential election, the tipping point state was Colorado, which Obama won by 5.4%, while he won the national popular vote by 3.9%. Thus, the electoral college bias was D+1.5, and so this market would have resolved to "D+1 to D+2" for that election.

In the 2020 presidential election, the tipping point state was Wisconsin (because Pennsylvania alone would have gotten Biden to 269 electoral votes; Wisconsin was needed to get him to 270). He won Wisconsin by 0.6% and the national popular vote by 4.4%. Thus, the electoral college bias was R+3.8, and so this market would have resolved to "R+3 to R+4".

(Faithless electors are irrelevant to the resolution of this market.)

[1] Defined as the percentage of the electorate in the state that voted for the Democratic candidate, minus the percentage that voted for the Republican candidate.

[2] Presumably Harris, but if someone else somehow becomes the Democratic nominee then the question is about them.

[3] 270 rather than 269 because I expect that an exact electoral college tie would be resolved in Trump's favor.