HOUSTON — Back in 2017, Jon Rosenthal, a mechanical engineer who’d been spurred into politics, like many Democrats, by Donald Trump’s election, was having drinks with a group of local party activists when one of them suggested that he run for office: Why not challenge the Republican who’d been holding onto his state House seat for more than 20 years?
Rosenthal was intrigued. And as he began considering his prospects, poring over demographic data on a laptop in his suburban...