BOSTON Democrats begin their national convention today united on many issues, but divided on how much to make of the Bush administration's refusal to push for an extension of the 10-year-old ban on assault weapons. The ban expires at midnight on Sept. 13, and Claire Buchan, a Bush spokeswoman, said he would sign an extension if Congress sends him one. But House GOP leaders said they won't schedule a vote unless Bush specifically asks. And they said that he hasn't done so. Sarah Brady of the Brady Campaign to Prevent Gun Violence said polls continue to show that three-quarters of...