Will eventually resolve to the party which won the popular vote for the race, as determined by the local, state, and ultimately federal governments.
In the case the race is called by the major news organizations, the market may optionally resolve early.
This market covers events up to and including Election Day, November 5: if Party A's candidate X wins the popular vote that day, but intervening events cause the next inaugurated Senator to be from Party B, the market will resolve to Party A.
If candidate X switches party affiliations, the switch will not affect the outcome if it is after the beginning of Election Day, local time.
From the latest Manifold newsletter:
Utah: Caroline Gleich (D) vs. John Curtis (R)
Expert forecasts: Solid R (Cook), Safe R (Sabato), Solid R (Inside Elections)
Poll results (538): Curtis+29, Curtis+30, Curtis+19
Election result in 2018, a Democratic wave year: R+32 (Romney vs. Wilson)
Election result in 2012, amid Obama's reelection: R+35 (Hatch vs. Howell)
Recent Utah presidential election results: Trump+20 (2020), Trump+18 (2016), Romney+48 (2012)
Commentary:
In 2018, Mitt Romney became the oldest freshman senator. He’s not running for reelection, and the Republican nominee this time is Rep. John Curtis. I couldn’t find any good fun facts about this race.