The most important national politicians today have more daughters than sons. The most important ones of all have only daughters. For instance:
John McCain (if you don't count his wife's children which he "adopted")
John Edwards (more surviving daughters than sons)
Al Gore
George Allen
Nancy Pelosi
Steny Hoyer (no sons)
Newt Gingrich (no sons)
John Boehner (no sons) -- beat heavily sonned Matt Blunt in an upset for House Majority Leader
Russ Feingold (no sons)
John Kerry (no sons)
Mr. & Senator Clinton (no sons)
Dick Cheney (no sons)
George Bush (no sons)
The only notable exceptions, Bill Frist and Dennis Hastert, have only sons. Coincidentally, both are often viewed as incapable of controlling their respective caucases.
The only explanation I can think of is that politicians without sons are less likely to be embarassed by their children's behavior (e.g. Duke Cunningham). But then, Jenna Bush has gotten into the headlines in an unfavorable way more than once. So I really have no idea.
Look for 2008 to be a contest between Russ Feingold and Newt Gingrich. You heard it here first.