TORONTO: President Obama has left little doubt that he stands against the forces of protectionism. Time and again, he has urged nations grappling with ailing economies to refrain from the temptation to erect trade barriers, warning that beggar-thy-neighbor policies would ricochet back to the instigator and sink everyone.

When Congress added a "Buy American" provision to the $787-billion economic stimulus package, Obama was quick to declare that such action would not violate trade pacts. Canada, the country's biggest trading partner, was relieved.

But the human desire to put the prosperity of one's own country ahead of others—especially when huge tax dollars are being spent to resurrect a domestic economy—may be just too strong for a U.S. president to extinguish. Anecdotal evidence is building that the provisions that require the use of U.S.-made products is spreading and preventing some Canadian suppliers from participating in U.S. infrastructure projects.

In May, IPEX Inc. of Toronto was horrified to learn that some plastic piping it supplied for a new health care center at the Camp Pendleton Marine Base in California had been...