I'm defining "decided by" a little weirdly, so please read the description before betting.

Arrange all of the states (+ congressional districts in Maine and Nebraska) from red to blue based on 2024 margin of victory (Trump's % of the vote - Harris' % of the vote). If Trump is the winner, start flipping the closest states (or congressional districts) that Trump won, one by one, until Harris wins the election. How many votes do you have to flip? (If Harris is the winner, do the symmetric thing.)

Example: in the 2004 presidential election, Bush won, and the closest states were Iowa (won by 0.67% or 10,059 votes), New Mexico (0.79% or 5,988 votes), and Ohio (2.11% or 118,601 votes). Flipping IA and NM wouldn't have been enough for Kerry, but flipping those plus OH would have been enough. Thus, the 2004 election was decided by 10059 + 5988 + 118601 = 134648 votes.

(Note that flipping Ohio by itself would have been enough for Kerry to win the election. However, because IA and NM were closer, my definition defines the 2004 election to have been decided by 134648 votes, not 118601.)

If I did the math right, this is how many votes the last six elections were decided by:

So this market would have resolves YES for 2000, 2016, and 2020 and NO for 2004, 2008, and 2012.

(Edit: see also this market, which defines "decided by" a little differently. I didn't notice it at the time of creation.)