In his column for the Washington Post on Tuesday, the influential conservative George Will provided intellectual fodder for the campaign among some Republicans to hang the Afghanistan war around the Obama administration's neck. Washington, he wrote, should “keep faith” with our fighting men and women by “rapidly reversing the trajectory of America's involvement in Afghanistan.” “Obama's war,” a locution one is now beginning to hear from other conservatives, is an expression of discontent that has been smoldering beneath the surface for several months.

The weakening public support for continuing the counterinsurgency campaign is not surprising. In the midst of an economic crisis people are tempted to draw inward. Add to that a general war weariness in the U.S. and the fact that the Afghanistan war is not going well right now-violence in Afghanistan is already far worse this year than last-and you have the makings of an unpopular conflict.

But the case of conservative opposition to the war in Afghanistan — as well as increasingly in Iraq — is symptomatic of something larger: the long history of political parties out of power advancing a neo-isolationist outlook. For example, Democrats were vocal opponents of President Reagan's support for the Nicaraguan contras and the democratic government in El Salvador, the U.S. invasion of Grenada, the deployment of cruise and Pershing missiles in Europe, and the forceful stand against the Soviet Union generally.

Many Democrats were also uneasy with or outright hostile to the policies of President George H.W. Bush. That included strong criticisms of the U.S. liberation of Panama and widespread Democratic opposition to the first Gulf War, which only 10 Senate Democrats voted to authorize.

The tables were turned in the 1990s: Then-House Majority Leader Tom DeLay called Kosovo “Clinton's war” and a majority of Senate Republicans voted against a bombing campaign, even after the Serbs had created half-a-million refugees in Kosovo and were on a path to destabilizing southern Europe. And, unlike today, this was not at a time of economic insecurity at home. Nor were we shouldering the military burden alone (18 other nations fought alongside us in the Balkans). Conservatives also argued that President Clinton's strikes against Saddam Hussein's Iraq in 1998 were meant to distract the nation's attention from the Monica Lewinsky scandal. In 2000, in a sharp rebuke of the Clinton administration's nation-building, Condoleezza Rice — then a top adviser to presidential candidate George W. Bush — said that the 82nd Airborne should not be walking kids to school.

In this decade, Democrats were fierce opponents of President Bush's Iraq policy, going so far as to declare the war lost and doing everything in their power to stop the surge — which turned out to be enormously successful — from going forward.

Our concern is that this tendency for the party out of (executive) power to pull back from America's international role and to undermine a president of the opposing party will gain strength when it comes to President Obama's policy on Afghanistan.

The president deserves credit for his commitment earlier this year to order an additional 17,000 troops for Afghanistan, as well as his decision to act on the recommendation of Gen. David Petraeus and Defense Secretary Robert Gates to replace the U.S. commander in Afghanistan with Gen. Stanley McChrystal.

These were tough and courageous decisions. The president's actions have clearly unsettled some members of his own party, who hoped he would begin to unwind America's commitment in Afghanistan. Mr. Obama not only ignored their counsel; he doubled down his commitment. There should therefore be no stronger advocates for Mr. Obama's Afghanistan strategy than the GOP.

The war in Afghanistan is a crucial part of America's broader struggle against militant Islam. If we were to fail in Afghanistan, it would have calamitous consequences for both Pakistan and American credibility. It would consign the people of Afghanistan to misery and hopelessness. And Afghanistan would once again become home to a lethal mix of terrorists and insurgents and a launching point for attacks against Western and U.S. interests. Neighboring governments — especially Pakistan's with its nuclear weapons — could quickly be destabilized and collapse.

Progress and eventual success in Afghanistan — which is difficult but doable — would, when combined with a similar outcome in Iraq, constitute a devastating blow against jihadists and help stabilize a vital and volatile region.

We also believe supporting the president's Afghanistan policy is politically smart for Republicans. For one thing, isolationist tendencies don't do well in American politics. Even in a war as unpopular as Vietnam, George McGovern's “Come Home, America” cry backfired badly. So has every attempt since then. There is no compelling evidence that the congressional GOP was politically well served in the 1990s by opposing intervention in the Balkans.

In addition, indifference or outright opposition to the war would smack of hypocrisy, given the Republican Party's strong (and we believe admirable) support for President Bush's post-9/11 policies, its robust support for America's democratic allies, and its opposition to rogue regimes that threaten American interests. Republicans should stand for engagement with, rather than isolation from, the world. Strongly supporting the president on Afghanistan would also be a sign of grace on the part of Republicans. We know all too well how damaging it was to American foreign policy to face an opposition that was driven by partisan fury against our commander in chief. Republicans should never do to President Obama what many Democrats did to President Bush.

Mr. Obama's policies shouldn't be immune from criticism; far from it. Responsible criticism is a necessary part of self-government. And we are particularly concerned about reports that retired Marine Gen. James Jones, Mr. Obama's national security adviser, told Gen. McChrystal earlier this summer not to ask for more troops and that the Obama White House is wary to offer what Gen. McChrystal says he will need to succeed.

We do believe, however, that Republicans should resist the reflex that all opposition parties have, which is to oppose the stands of a president of the other party because he is a member of the other party. In this instance, President Obama has acted in a way that advances America's national security interests and its deepest values. Republicans should say so. As things become even more difficult in Central Asia, it's important to keep bad political patterns we have seen before from re-emerging.

Mr. Senor is an adjunct senior fellow at the Council on Foreign Relations. Mr. Wehner is a senior fellow at the Ethics and Public Policy Center. They both served as officials in the administration of George W. Bush.