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:
Delaware: Lisa Blunt Rochester (D) vs. Eric Hansen (R)
Democrat win chances per models: >99% (The Hill), 98% (538), 100% (Split Ticket)
Expert forecasts: Solid D (Cook), Safe D (Sabato), Solid D (Inside Elections)
Polling average (538): Not enough polls
Election result in 2018, a Democratic wave year: D+22 (Carper vs. Arlett)
Election result in 2012, amid Obama's reelection: D+37 (Carper vs. Wade)
Recent Delaware presidential election results: Biden+19 (2020), Clinton+11 (2016), Obama+19 (2012)
Commentary:
In a state like Delaware, which has two Senate seats but only one House seat, which body is it more impressive to be elected to? It’s quite the philosophical quandary. Anyway, I needed something to fill the space — god it’s like 2am as I write this — because Delaware-at-large Rep. Lisa Blunt Rochester will now become Delaware Sen. Lisa Blunt Rochester.