The GOP is mostly defending favorable turf

Midterm Senate seats defended by the president's party, sorted by how much the state leaned toward or away from that party in the last presidential election, 1994 to 2026

Figures from 1994 to 2022 based on incumbent party at the time of the November election. Figures for 2026 based on current incumbency in seats on the ballot in the midterm election. Independent incumbents are included with the party they caucused with. The lean of a state is its margin in the last presidential race vs. the national popular vote margin.
Source: Dave Leip's Atlas of U.S. Presidential Elections, State Election Results, U.S. Senate