Overview:
"The Teamsters" are considered one of the most powerful and politically active unions in the history of the world. They have endorsed presidential candidates from both major parties, and have sat out of elections such as Kennedy vs. Nixon (1960). This cycle, they have donated evenly to both the RNC and the DNC, and have met privately with Donald Trump as recently as late January.

This market resolves to whichever answer reflects the public and explicit endorsement made by the International Brotherhood of Teamsters for the United States presidential election in 2024. This announcement is likely to be made live by union executive leadership through official channels, and this market will not resolve based on vibes, political leaning of membership, or unofficial statements by leadership. Feel free to ask clarifying questions in the comments!

Pundits have begun speculating:

Edge cases:
Third Party will also cover anyone else other than the Democrat or Republican candidate, even if that person doesn't technically represent a political party.

Trump or Biden can be replaced with their VP/backup party candidate if they are replaced before the Teamsters make an endorsement. (EDIT: this happened!)

No One covers both an explicit announcement that the Teamsters aren't endorsing, as well as the union remaining silent on the issue through election day.
In the event that the Teamsters are disolved or otherwise indisposed, this will also resolve to No One.

In a very unlikely Teamsters civil war or wildcat strike scenario, this market will resolve to either the candidate who receives a clear majority of Local endorsements, or to No One.