Republican presidential candidate Donald Trump defended his position on the Iraq war during an interview with CBS's Face the Nation on Sunday, Feb. 14 following CBS's GOP debate in Greenville, South Carolina on Feb. 13. (Reuters)

It's hard to settle on just one vulnerability from Donald Trump's performance at Saturday night's Republican debate in Greenville, S.C. But there's only one you can buy on Etsy. For $12, you too can own a shirt that reads "Iraq: We Were Winning When I Left!"

For slightly more, you can find one on Amazon. Poke around on military web forums, and you can find people seeking merchandise that will let them make this basic point: America had pacified Iraq, and then Barack Obama decided to lose the war.

Trump's argument against the war in Iraq -- which he did not make in 2003, or at least not as loudly as he claims -- has not changed during his campaign for president. He has always pronounced it a disaster. "The war in Iraq, we spent $2 trillion, thousands of lives, we don't even have it," he said Saturday night, during one of many Trump debate riffs that were interrupted by boos.

Mainline conservative anger at Trump today has focused on whether he "blamed" George W. Bush for 9/11. But the entirety of the debate on foreign policy underscored how the Iraq war will be viewed if someone like Jeb Bush wins the nomination, or if Trump does. The rehabilitation of George W. Bush's reputation depends on whether viewers blame him or Obama for unrest in the Middle East.

Jeb Bush, like most Republicans, has argued that the Iraq war was basically won until Obama made political decisions to lose it. "ISIS didn’t exist when my brother was president," Bush said last summer. "Al Qaeda in Iraq was wiped out when my brother was president. There were mistakes made in Iraq for sure, but the surge created a fragile but stable Iraq that the president could have built on."

If the argument sounded familiar, it was because hawks had said similar things about Vietnam, for decades. It began as soon as North Vietnam conquered South Vietnam; within months, William Westmoreland was explaining how the war was only lost when Washington politicians gave up on it.

That theory really breathed in 2004, when now-Secretary of State John Kerry won the Democratic nomination on a bet that a Vietnam veteran could uniquely degrade Bush's advantage on post-9/11 national security. Democrats didn't appreciate the uniqueness of Kerry's resume -- he served in Vietnam, then became a prominent anti-war activist.

The Swift Boat Veterans for Truth ad campaign is remembered for its factual challenges to Kerry's war record, but its most powerful ad was more tribal. In "Sellout," clips of Kerry reading examples of alleged war crimes by American soldiers were played alongside angry veterans saying that Kerry helped them lose the war. "John Kerry gave the enemy for free what I, and many of my comrades in North Vietnam, in the prison camps took torture to avoid saying," said one veteran. "It demoralized us.”

If five of the remaining six Republican candidates win the nomination, they'll be well-positioned to argue that Obama lost Iraq. They can say as much to Sen. Bernie Sanders (who consistently voted to end the war) or former secretary of state Hillary Clinton. Jeb Bush, in particular, has built this case alongside Vietnam and Iraq veterans who often introduce him at rallies. He may well deploy George W. Bush to say this Monday in North Charleston.

But a contest between Trump and any Democrat would pit against one another two people who agree that the Middle East was basically stable until the folly of removing Saddam Hussein. It would align Republican voters with what Trump has argued through the campaign -- that the last Republican president was a disaster and his advisers are discredited.