An alert this week from backers of Judge John Roberts cautions not to take seriously Democratic complaints that they cannot stop his confirmation. A three-page memo sent to thousands of conservatives across the country warns that the assault on President Bush's first Supreme Court nominee is yet to come. A major reason cited for this belief is the man back at Sen. Edward M. Kennedy's side on the Senate Judiciary Committee: James Flug. ''It is hard to fathom Mr. Flug coming back to Capitol Hill after 30 years of private practice for anything other than a bitterly tough confirmation fight,''...