History. 2 of 5: Oklahoma 1926 by 11.3 and Louisiana 2014 by 11.8. The other losses were by 3.4 and 5.6, both on one day in 1918.
Hunch. September polls showed a tie or Collins 3 points ahead, far from the 11 and 12 point defeats in Oklahoma and Louisiana. Editorial scoring note: Score the final election result, including a runoff, rather than a fixed 28-day window. Louisiana 2014 therefore counts as a loss despite its 32-day runoff. This gives four losses in five elections and two losses by more than eight points. The existing market resolution rules are unchanged.
The margin market resolves on the official Maine Secretary of State result, using the final round if ranked-choice counting runs: YES if Jackson's share minus Collins's share is more than 8.0 points, NO if it is 8.0 or less or Collins wins. The AP count, Reuters and The Guardian are the backup.
We looked for cases where a senator from the president's party ran again in a midterm in a state that had voted for the other party for president, while chairing a Senate committee and facing a governor from the other party. We found five since 1918, all with the president under 50% approval or not yet measured. Only one kept the seat.