Two years ago, a 26-year-old immigration lawyer named Jessica Cisneros came within 3.6 percentage points of pushing out the longtime Democratic congressman here, running aggressively on the progressive vision of the national liberals who had bankrolled her insurgent campaign.

This time around, at the ripe age of 28, she’s scorching the already brown earth of South Texas, attacking Representative Henry Cuellar not so much for his conservative policy positions, but for being what she describes as a corrupt politician — rich, out of touch with his poor constituents, and quite possibly a felon…

In South Texas, another test is developing over the power of identity politics and whether liberals can answer the fears that conservatives are stoking about “open borders,” “critical race theory” and rising crime. In the primary campaign for Texas’ 28th district, it is Mr. Cuellar’s experience versus Ms. Cisneros’s storytelling: the powerful and connected versus the underdogs, the community, the “pueblo.”