Complex sentence: Mannix ran a vigorous campaign, but was narrowly defeated by Mike Erickson, who went on to lose to Kurt Schrader in the general election due in part to personal issues that Mannix brought up in the primary.
Split into simpler sentences: Mannix ran a vigorous campaign, but was narrowly defeated by Mike Erickson. Erickson lost the general election to Kurt Schraeder due in part to personal issues that Mannix brought up in the primary.