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.
Analysis from the Manifold newsletter:
Michigan: Elissa Slotkin (D) vs. Mike Rogers (R)
Democrat win chances per models: 68% (The Hill), 78% (538), 74% (Split Ticket)
Expert forecasts: Tossup (Cook), Lean D (Sabato), Tilt D (Inside Elections)
Polling average (538): Slotkin+4
Election result in 2018, a Democratic wave year: D+7 (Stabenow vs. James)
Election result in 2012, amid Obama's reelection: D+21 (Stabenow vs. Hoekstra)
Recent Michigan presidential election results: Biden+2.8 (2020), Trump+0.2 (2016), Obama+10 (2012)
Manifolder commentary:
Plasma Ballin’: Republican internals show Slotkin up by 8.
Conflux commentary:
Here, a battle between two politicians who once represented the same congressional district: Democrat Elissa Slotkin has represented Michigan’s 7th since 2019, when it was named the 8th. Republican Mike Rogers was in office from 2001 to 2015. Speaking of residency, both candidates have controversies: Rogers “moved to Florida because of some business opportunities,” making him a potential carpetbagger in Michigan, while Slotkin was accused of staying in a lobbyist’s house.
As Sen. Debbie Stabenow is retiring, this race has no incumbent — which would push it toward being a true tossup. And unlike other races in this set, the Republican does have political experience. However, Michigan is simply a bluer state than Pennsylvania or Wisconsin, and the polls show Slotkin with a modest lead.