AT SATURDAY'S New Hampshire debate, Democratic candidates were confronted with a question that they have been ducking for some time: Can they concede that the "surge" of U.S. troops in Iraq has worked? All of them vehemently opposed the troop increase when President Bush proposed it a year ago; both Hillary Rodham Clinton and Barack Obama introduced legislation to reverse it. Now it's indisputable that the surge has drastically reduced violence. Attacks have fallen by more than 60 percent, al-Qaeda has been dealt a major blow, and the threat of sectarian civil war that seemed imminent a year ago has receded. The monthly total of U.S. fatalities in December was the second-lowest of the war.