One problem with the conversation around political polarization is that it can imply that polarization is a static, singular thing. That our divisions are fixed and unchanging. But that’s not how it is at all. The dimensions of conflict change, and they change quickly. In the Obama era, Republicans mobilized against government spending and deficits but didn’t think much about election administration. Now, a trillion-dollar infrastructure package has passed the Senate with bipartisan support, but the divisions over democracy and voting access are deep. Lilliana Mason is one of the political scientists I’ve learned the most from in recent years. Her 2018 book, “Uncivil Agreement: How Politics Became Our Identity,” is, in my view, one of the most important political books of the last decade. But it’s been a tumultuous three and a half years since it was published. And Mason has continued to pump out important new work on political identity, how support for Donald Trump differs from that of other Republicans, when Democrats and Republicans believe political violence is justifiable and even necessary, and much more. And so I wanted to have Mason on the show to discuss how her thinking has changed in recent years and, in particular, which identities and interests she thinks are at the center of our political collisions today. Mentioned: Uncivil Agreement by Lilliana Mason "Who's At the Party? Group Sentiments, Knowledge and Partisan Identity" by John Victor Kane, Lilliana Mason and Julie Wronski "Activating Animus: The Uniquely Social Roots of Trump Support" by Lilliana Mason, Julie Wronski and John Victor Kane "Fear and Loathing Across Party Lines: New Evidence on Group Polarization" by Shanto Iyengar and Sean J. Westwood The Sum of Us by Heather McGhee Book Recommendations: Reconstruction by Eric Foner Black Reconstruction in America 1860-1880 by W. E. B. Du Bois Children of Blood and Bone by Tomi Adeyemi The City We Became by N. K. Jemisin You can find transcripts (posted midday) and more episodes of "The Ezra Klein Show" at nytimes.com/ezra-klein-podcast, and you can find Ezra on Twitter @ezraklein. Thoughts? Guest suggestions? Email us at [email protected]. “The Ezra Klein Show” is produced by Annie Galvin, Jeff Geld and Rogé Karma; fact-checking by Julie Beer and Michelle Harris; original music by Isaac Jones; mixing by Jeff Geld, audience strategy by Shannon Busta. Special thanks to Kristin Lin.