HOUSTON — Nationally, the Democratic Party, which gave indispensable assistance (“Basket of deplorables”! ) to the election of today’s president, seems intent (“Impeach! ”; “Abolish ICE! ”; “Free stuff! ”, “I am Spartacus! ”) on a repeat performance. Here, however, in the 7th Congressional District, in what might turn out to be the year’s most instructive House race, Democrats seem serious about winning, and if they do with Lizzie Fletcher, they will have a template for 2020 nationally.