No one knows better than Bill and Hillary Clinton that politics can be a full-contact sport.

That's why it was surprising to hear the former president cry foul in an effort to defend his wife after a Democratic candidates debate in Philadelphia.

Opponents pounced after Hillary waffled when asked whether she supported a plan by New York Gov. Eliot Spitzer to issue driver's licenses to illegal immigrants.

Bill Clinton said that putting candidates on the spot during debates could leave "Democrats vulnerable to a swift-boat kind of ad in the general election."

Swiftboat is a term that arose during the 2004 presidential campaign when a group of Vietnam veterans stepped way over the line — even by lowdown political mudslinging standards — by using television advertisements to question the patriotism and Vietnam heroics of Sen. John Kerry, D-Mass.

Sen. Barack Obama, D-Ill., said he was "stunned" that Clinton compared the criticisms of his wife to the swift-boat ads. Sen. Christopher Dodd, D-Conn., called Clinton's comparison "over the top" and "outrageous."

Clinton knows better. After all, his political operatives invented the "war room" to respond to attacks during his presidential campaigns. But this time he went too far.

He made a poor choice in trying to portray his wife as a victimized woman. After all, it was Hillary who once famously said she was no Tammy Wynette standing by her man.

Bill Clinton also muddied the different degrees of acceptable political rhetoric by cavalierly throwing around the swiftboat label. Aggressive but civil questioning on the issues is fair game — and good for the country's political discourse.

If the Clintons thought the recent round of attacks on Hillary were tough, just wait until the vast right-wing conspiracy takes flight.