In his thoughtful review of my Movements and Parties, Sam Popkin recognizes the value of bridging the divide between scholars of social movements and political scientists who study voters or legislators. His review is so gracious that it would be churlish of me to complain about his criticisms. Rather, I focus on three ways in which Popkin urges me to take further my arguments concerning the party/movement nexus. Drawing on his long experience in party campaign work and as a consummate political analyst, Popkin argues that first, movements sometimes jog parties to move beyond their existing commitments, a point I could have made more pointedly; second, “movements already embedded in a party can block change,” a point I failed to make; and, third, “just as some wars break states, some movements break parties,” the most far-reaching of his amendments.

Movements can jog parties to move beyond existing commitments: In chapters on Wilson’s reluctant support of the suffrage movement, the New Deal’s partnership with the CIO, and the New Right’s defeat of the moderate wing of the GOP, Movements and Parties supported this argument but could have gone further. For example, after the 1960s, the Democrats began to absorb the messages of the cultural Left. Similarly, Trumpism shifted the GOP’s center of gravity far to the right. When movements jog parties to move beyond existing commitments, they can even convert sectors of the party—as Trump was able to do with opportunists like Lindsey Graham.

Movements embedded in a party can block change: The primary reforms of the 1970s helped lead to the move of Christian conservatives into the grassroots of the GOP, where they helped impede that party’s ability to reach out to new constituencies. Once entrenched, these groups can harden into internal interest groups, like the teachers’ unions that have been a drag on the Democrats’ ability to propose educational reforms.

Just as war can break states, movements can break parties: This is a claim that movement scholars—who sometimes appear to be cheering for movements— have failed to investigate. Think of the inability of the Republicans after their losses in the 2008 and 2012 elections to take seriously that they risked becoming a party of mostly white, majority-male, and increasingly older voters. When a demagogic businessman with a gift for inflated rhetoric appealed to them in the 2016 election, the party risked “cracking up”—to adopt the language of Popkin’s book.

These extensions have led the party system to turn into a “traffic jam.” They lead Popkin to worry about the parties’ ability to adapt in the future. To the extent that movements have played a role in creating this situation, scholars of social movements and parties should take his concerns seriously. I certainly will!