NEW YORK (AP) — Donald Trump’s campaign has spent the last year-and-a-half viciously attacking Joe Biden, ridiculing his policies, mocking his fumbles and relishing a rematch they felt they were winning. But they have also spent weeks preparing for the possibility that Biden might exit the race, readying a bevy of attacks against Vice President Kamala Harris that they unleashed as soon as Biden made his stunning announcement Sunday that he would step aside. Biden soon after endorsed Harris, who was quickly winning support from Democrats to be the party’s nominee. The shakeup less than four months before Election Day lays out new challenges for Trump’s team, which had until recently been focused on contrasting the former president’s vigor and mental acuity with Biden’s.