Afghanistan was supposed to be the good war -- the one Democrats said we should be fighting instead of Iraq. We heard it over and over again during the presidential campaign, as if to exorcise the image that a Democrat wasn't tough enough to assume the role of commander in chief. Candidate Obama repeatedly called the war in Iraq "a dangerous distraction" from the fight we should be waging and promised to "tak(e) the fight to al-Qaida in Afghanistan and Pakistan." But with a new poll out showing that a majority of Americans now think the war in Afghanistan isn't worth fighting, it won't be long before Democrats decide to turn tail.

Afghanistan has always been as difficult a challenge as Iraq, if not more difficult. It is both larger and more populous than Iraq, with a population that is less educated, more tribal, and used to repelling foreign invaders over the centuries. The war in Afghanistan was originally conceived as a necessary war after nearly 3,000 Americans lost their lives in an attack planned there, so rooting out the Taliban supporters of al-Qaida was viewed as justified. Now, however, some Americans have changed their minds.

A Washington Post-ABC News poll taken in mid-August shows that a bare majority, 51 percent, now question whether the fight in Afghanistan has been worth waging. But the poll reflects an interesting divide. Democrats are far more skeptical than Republicans. Seven in 10 Democrats now say the war hasn't been worth the costs, while 70 percent of Republicans say it is worth fighting, with Independents evenly split 49-49 percent.

The U.S. has doubled the number of American troops in Afghanistan, which now stands at 68,000, but more are needed. Even with the additional 33,000 NATO troops in Afghanistan, the numbers of security forces in the country are far smaller than similar forces in Iraq. And the fact is, Americans only support wars they think they're winning. Public opinion turned against the war in Iraq when Americans believed it could not be won.

As National Council on Foreign Relations senior fellow Max Boot observed recently at Commentary Magazine's Contentions, "the same dynamic applies as that which held in Iraq and in most of our other wars: the public is skeptical because they don't see enough signs of progress. ... Only by adequately resourcing the war effort and pursuing an effective counterinsurgency strategy can the U.S. armed forces make the progress necessary to raise public support for the war effort and win what President Obama has just described as a 'war of necessity.'"

It is easy to believe that the danger posed by terrorism is waning -- we have not had an attack on American soil in nearly eight years, after all. But it would be foolhardy to believe that those intent on the destruction of our way of life have simply moved on or are so weakened that they pose little threat to us.

Iraq has been racked with violence in the last several days, with lethal attacks aimed at toppling the government there. The remaining American forces have been left on the sidelines unable even to assist the wounded and dying without being directly asked to do so by Iraqi leaders. And the fight in Afghanistan is proving difficult, especially since so few Afghan forces are trained to help, and NATO troops have shown themselves, as Boot points out, "unwilling either to fight or to provide the resources for fighting effectively."

President Obama sold the American electorate on the notion that he was tough enough to take on America's enemies. But if he wants to keep America safe, he will have to challenge his own supporters -- Democratic voters and members of Congress -- to support his efforts in Afghanistan. He's done the right thing to date by increasing U.S. troops, but he has to convince the public that this war is winnable in order to continue the fight. And the only way to do that is for the president to give his military commanders in the field the added resources and troops they need to prevail, even if it proves temporarily unpopular. It remains to be seen whether the president will do so.