By BILL BARROW ATLANTA — With President Joe Biden ending his reelection bid and endorsing Vice President Kamala Harris, Democrats now must navigate a shift that is unprecedented this late in an election year. Democrats are set to hold their convention in Chicago on Aug. 19-22. What was supposed to be a coronation for Biden now becomes an open contest in which nearly 4,700 delegates will be responsible for picking a new standard-bearer to challenge Republican Donald Trump in the fall. The path ahead is neither easy nor obvious, even with Biden endorsing Harris.