2. Conor Lamb, the Democrat in a special House election in southwestern Pennsylvania, scored an extraordinary razor-thin upset in what had been a Trump stronghold.

Mr. Lamb, a 33-year-old Marine Corps veteran from a local political dynasty, echoed traditional Democratic themes about union rights and economic issues in his campaign.

But he also rejected the leadership of Representative Nancy Pelosi, welcomed support from people who voted for President Trump and took a more conservative position on the hot-button issue of guns.