What happened to Rob Lowe's character in the car accident on the TV show Brothers and Sisters?

In the TV show "Brothers and Sisters," Rob Lowe's character, Senator Robert McCallister, is involved in a car accident that ultimately leads to his death. This tragic event occurs during the show's fifth season and serves as a pivotal moment for the storyline, deeply affecting his family and the other characters. His character's departure marked a significant turning point in the series.