WASHINGTON - Ever the cautious centrist, Sen. Bill Nelson sometimes gets hit from all sides. When the Florida Democrat harshly criticized the Bush administration for the Iraq war in November, telling a Sarasota fundraiser, ''We're in a heck of a mess,'' Republicans tried to liken him to Howard Dean, the Democratic national chairman.Early last year, some liberals grumbled when Nelson, unlike most of his Democratic colleagues, voted to confirm Alberto Gonzales as attorney general. Nelson's reasoning was simple: Gonzales was fair and easy to deal with on Florida judges. And despite pressure from pro-immigration activists, Nelson has not taken a...