Are Americans weaving their political views more tightly into the fabric of their self-identity over time? If so, then we might expect partisan disagreements to continue becoming more emotional, tribal, and intractable. Much recent scholarship has speculated that this politicization of Americans’ identity is occurring, but there has been little compelling attempt to quantify the phenomenon, largely because the concept of identity is notoriously difficult to measure. We introduce here a methodology, Longitudinal Online Profile Sampling (LOPS), which affords quantifiable insights into the way individuals amend their identity over time. Using this method, we analyze millions of "bios" on the microblogging site Twitter over a 4-year span, and conclude that the average American user is increasingly integrating politics into their social identity. Americans on the site are adding political words to their bios at a higher rate than any other category of words we measured, and are now more likely to describe themselves by their political affiliation than their religious affiliation. The data suggest that this is due to both cohort and individual-level effects.