Congressional Democrats are cautiously coming to one conclusion: We’re no longer doomed. President Joe Biden’s exit from the presidential race breathed new life in the fight for control of Congress, and the party’s candidates said they feel like they once again have a real chance at winning. Within 24 hours, many of the most vulnerable Democratic incumbents coalesced behind Vice President Kamala Harris as the nominee, eager to project unity after weeks of chaos. Almost no one is...