How Mayra Flores flipped a Rio Grande Valley congressional seat and gave Republicans hope for a new era in South Texas

After a redistricting proposal made Texas’ 34th Congressional District more blue last fall, the top Republican candidate for the seat, Mayra Flores, traveled to the state Capitol in Austin to plead with lawmakers to reconsider.

It seemed, she said, that despite all the new Republican talk about competing in South Texas, the GOP map-drawers were “sending the message of not really caring about” voters there, depriving them of a competitive district.

But lawmakers were unswayed and eventually passed a map that transformed the 34th District from one that President Joe...