New political issues and opportunities lead new actors into contentious politics. This article studies one such case: transnational migrants making claims and engaging in collective action when traversing state borders. As global migration flows and accompanying political backlash has grown since the mid-2010s, borders have increasingly become sites of contention between groups of migrants seeking entry and state agents attempting to refuse it. Media coverage and elite discourse also has focused on contentious border crossings, with implications for public attitudes toward migration. In this setting, public attitudes toward migrants should vary based on the migrants’ tactics and characteristics. We expect migrants engaging in nonviolent resistance to security forces will win more public support than those engaging in violence. Migrant characteristics – claims or motives for migration and ethnic identity – should also affect support. Survey experiments in the United States and Mexico containing fictionalized vignettes of a contentious event at the countries’ shared land border show strikingly similar results: migrant nonviolent resistance, compared to violent confrontations, reduces support for deportation and increases beliefs that migrants contribute to society. These effects are consistent across party lines and border proximity. Neither migrants’ claims nor migrants’ ethnic identity affect public support in the context of a contentious event.