WASHINGTON -- In an exchange that suggested a growing dynamic in the 2000 Presidential race, President Clinton seized the occasion of Elizabeth Dole's departure from the Republican field on Wednesday to take a swipe at the front-runner, Gov. George W. Bush of Texas, and the Bush campaign promptly returned the favor. Referring to Mrs. Dole's stated frustration with her campaign's financial disadvantage, the President said, "I think that part of what you see is the fact that Governor Bush is the first candidate in the history of the modern era, when we've had Federal financing, who's given it up ...