According to Polymarket:

How do we know?

Polymarket can tell us what the odds of a candidate winning their party's nomination, and their odds of winning the presidency.

You can combine these numbers to figure out how likely they are to win the presidency, if they manage to win the primary.

Imagine you know a player has a ten percent odds of scoring ten points in a game, but due to a recent injury they only have a twenty percent odds of making it on to the field at all. We need to figure out the odds that they score ten points assuming they get onto the field. To do this we divide the chance of scoring ten points by the chance of making it onto the field, to learn that if they do get onto the field, they have a 50% chance of scoring ten points.

For The Nerds

• The math is simple, it's just P(Presidency | Nomination) = P(Presidency) / P(Nomination).
• You might also ask, how can this Democrat have a 75% chance of winning and this Republican have a 50% chance of winning, it adds up to over 100%! That's because the Republican has a 50% chance on average, he might be weak against that Democrat but strong against many other democrats.
• Note: This site doesn't show anyone with a less than 5% odds of winning the whole thing.