This resolves to whichever of the four candidates above (the ones being tracked by FiveThirtyEight) suspends their campaign for the GOP presidential nomination first, provided that it happens before June 8. If no one leaves by June 8 (the date of the last caucuses), resolves to that option.

Note: If the Supreme Court rules that Trump is ineligible to run, I'll count that as suspending his campaign. A ruling by a lower court wouldn't count, since it could be appealed.

I would also count it as leaving the race if one of the candidates died.