There are notable primary elections happening tonight in Nebraska, Idaho, Oregon, and Pennsylvania, but the biggest focus is on Pennsylvania because it stands to become a case-study in what happens when judges strike down voting maps that were gerrymandered by conservatives.
The assumption is that Democrats are now going to do a lot better in PA, picking up three Congressional seats—and maybe as many as six. The New York Times (perhaps getting a little jinxy) calls it "a sweet down payment on [Democrats'] goal of taking control of the House of Representatives in November."
Also, notes ABC News, the primary in PA is "a test for Democratic women." Right now the entire US House delegation from PA is male (not to mention overwhelmingly Republican).
But women are running in all but five of the state's 18 House districts. Slate says tonight is the first big test of the idea that "women, enraged by the ascent of Donald Trump and sick of watching men in power drop the ball, are poised to flood the ballots this year, winning seats in public office in record numbers."
You can watch the results come in right here.