This market resolves to the favorability percentage among Republicans according to 538:

https://projects.fivethirtyeight.com/polls/favorability/donald-trump/r/

The exact metric measured is "favorable" (and not "net favorable"). On February 8, 2024, Trump's "favorable" according to 538 is 78.5%. (His "unfavorable" is 19.5%, and his "net favorable" is +59.0%.)

The market will close at the end of 2024. The resolution decision will be made in 2025 as soon as I am able to get a favorability number from 538 for 1/1/2025. (The data thus might exclude surveys from 2024 that have not yet been added to 538, but the resolution will nonetheless stand.)

If 538 no longer aggregates data on Trump's favorability among Republicans at the time of Trump's death, then a suitable replacement will be found (with consultation from the community). The core of the idea is to measure the general approval and loyalty of Republicans toward Trump over time.