WASHINGTON (Reuters) - Dozens of interest groups on the right and left are poised to spend millions of dollars and mobilize thousands of activists in an expected fight over a U.S. Supreme Court vacancy that could tip the court's balance on hot-button social issues like abortion. With the retirement of ailing Chief Justice William Rehnquist widely anticipated, if far from certain, when the court ends its term next week, advocacy groups are planning for all possibilities and researching a long list of President Bush's potential replacement nominees. Both sides have extensive battle plans that feature the trappings of a modern...