BANGOR, Maine — Maine Democrats chose Troy Jackson, a progressive former state legislator, to replace Graham Platner as their Senate candidate at a convention Saturday that followed weeks of internal turmoil in a key battleground state.Jackson’s victory, a vote of 566-5, came on the first ballot of a pop-up state convention the Maine Democratic Party organized in the 17 days since Platner, who faced a rape accusation he denies, dropped out of the race. The coronation of Jackson, who has spent a quarter-century in Maine state politics, completes a tumultuous process since Platner was forced from the race. Jackson will...