Incumbent Sen. Jim Risch holds a commanding 98% implied probability on Polymarket as the Idaho Republican Senate primary approaches on May 19, driven by his three-term tenure, strong party establishment backing—including reported support from President Trump—and challengers' limited name recognition and fundraising. Joe Evans, a U.S. Army veteran and political newcomer, trails at 2% alongside minor candidates Denny LaVé and Josh Roy, with no recent polls indicating competitive traction despite absentee ballots now circulating. Historical base rates favor incumbents in safe-state primaries absent scandals, though late-breaking health issues for the 83-year-old Risch, a voting surge for challengers, or unforeseen endorsements could narrow the gap in the final days.