We investigated U.S. citizens’ politicization (i.e., switching from not self-defining to self-defining as an active political party supporter) during the 2012 U.S. Presidential Elections. We used a novel identity content approach to explore qualitative changes in overlap between personal and politicized identity traits. We collected longitudinal data from a community sample of U.S. citizens (N = 760), tracking whether and how personal and politicized identity content developed: two months before (T1), immediately before (T2), and 2 months after (T3) the election. We explored a subsample of participants who met inclusion criteria (n = 115), comparing 87 participants who did not politicize with 28 participants who self-labeled as unpoliticized at T1, but politicized at T2/T3. Results confirmed hypotheses: Only politicizers showed greater integration between their personal and politicized identity content over time; moreover, identity content was a significant positive predictor of politicization and action engagement. We discuss the value of identity content for politicization research.