Karl Rove, the president’s chief political adviser, is struggling to steer the Republican Party to victory this fall at a time when he appears to have the least political authority since he came to Washington, party officials said. Mr. Rove remains a dominant adviser to President Bush, administration officials say. But outside the White House, as Mr. Bush’s popularity has waned, and as questions have arisen among Republicans about the White House’s political acumen, the party’s candidates are going their own way in this difficult election season far more than they have in any other campaign Mr. Rove has overseen....