Erika Kirk, leader of the powerful conservative youth organization, endorsed him on opening night of its annual AmericaFest convention, drawing cheers from the crowd. But the four-day gathering revealed more peril than promise for Vance or any other potential successor to President Donald Trump, and the tensions on display foreshadow the treacherous waters that they will need to navigate in the coming years. The “Make America Great Again” movement is fracturing as Republicans begin considering a future without Trump, and there is no clear path to holding his coalition together as different factions jockey for influence. “Who gets to run...