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 my latest Manifold newsletter:
Tennessee: Gloria Johnson (D) vs. Sen. Marsha Blackburn (R)
Expert forecasts: Solid R (Cook), Safe R (Sabato), Solid R (Inside Elections)
Poll results (538): Blackburn+23, Blackburn+23, Blackburn+20
Election result in 2018, a Democratic wave year: R+11 (Blackburn vs. Bredesen)
Election result in 2012, amid Obama's reelection: R+34 (Corker vs. Clayton)
Recent Tennessee presidential election results: Trump+23 (2020), Trump+26 (2016), Romney+21 (2012)
Commentary:
In 2018, Taylor Swift gave a surprise endorsement to Phil Bredesen, the popular former governor of Tennessee who was making a longshot bid for Senate. While Bredesen outperformed the partisan lean of Tennessee, he still lost by 11 to Congressman Marsha Blackburn. This time, the Democratic nominee is State Rep. Gloria Johnson, who seems interesting but is not as well-known. It’s hard to imagine her even doing better than Bredesen.