When they weren’t jabbing at each other over health care and Iraq, Barack Obama and Hillary Clinton spent a good chunk of their debate last night arguing over which of them is the strongest critic of the North American Free Trade Agreement. Both declared that they would withdraw the United States from the agreement if Canada and Mexico did not agree to inserting “enforceable” labor and environmental standards into the agreement.

Talk about a non-starter. It is unlikely that our two neighbors would agree to reopen a 14-year-old agreement that has worked well for all three nations. [You can read my assessment of NAFTA here.] In effect, Obama and Clinton will be asking our two neighbors to bend their national labor and environmental standards to the demands of the U.S. Congress under threat of trade sanctions. Where exactly is the upside for Canada and Mexico in such a request?

Of course, there is no upside. So the only motivation will be the threat that the United States will unilaterally withdraw from NAFTA. That, of course, would result in the re-imposition of tariffs on trade with our two most important trading partners. And because Mexican tariffs on imports from the rest of the world are significantly higher than U.S. tariffs, U.S. exporters to Mexico would face a much steeper tariff increase than Mexican exporters to the United States. By withdrawing us from NAFTA, the Democrats would transform what has truly become a “level playing field” of zero tariffs into one tilted against U.S. exporters.

And even if the U.S. government were able to demand that Mexico impose new and tougher environmental and labor restrictions on its producers, there is little reason to believe that goods now made in Mexico would be soon be produced in Youngstown, Ohio, and elsewhere in the United States. The far more likely scenario is that producers in Mexico would shift production to China, Vietnam, and other lower-cost producers.

Finally, consider the foreign policy implications of threatening to withdraw from NAFTA. The Democratic candidates have been critical of the Bush administration for its checkered record of winning friends abroad. But have the Clinton and Obama campaigns considered how our friends in Canada and Mexico will react to the heavy-handed demand that they re-write their domestic labor and environmental laws under threat of face tariff retaliation from Uncle Sam?

This would confirm the worst fears of our closest neighbors.