Party activists have played a leading role in “conflict extension”—the polarization of the parties along multiple issue dimensions—in contemporary American politics. We argue that open nomination systems and the ambitious politicians competing within those systems encourage activists with extreme views on a variety of issue dimensions to become involved in party politics, thus motivating candidates to take noncentrist positions on a range of issues. Once that happens, continuing activists with strong partisan commitments bring their views into line with the new candidate agendas, thus extending the domain of interparty conflict. Using cross-sectional and panel surveys of national convention delegates, we find clear evidence for conflict extension among party activists, evidence tentatively suggesting a leading role for activists in partisan conflict extension more generally, and strong support for our argument about change among continuing activists. Issue conversion among activists has contributed substantially to conflict extension and party commitment has played a key role in motivating that conversion.