When Rep. Alexandria Ocasio-Cortez (D-New York) met with key Democrats last month to make her case to help lead a committee that will be critical in countering President-elect Donald Trump, she faced sharp questions from members of her party. As Ocasio-Cortez lobbied the powerful Steering Committee that decides panel assignments, Rep. Linda T. Sánchez (D-California) pressed her on her past decisions to endorse primary challengers against Democratic incumbents. Rep. Henry Cuellar (D-Texas), one of the incumbents Ocasio-Cortez tried to defeat, followed up by asking why Democrats should elevate her given her previous animosity toward her less-liberal colleagues, according to two...