Lisa Desjardins:

At risk is a key relationship on both sides. Canada is the United States' largest export market overall. It's also the largest market for American agricultural goods.

Since the North American Free Trade Agreement went into effect in 1994, exports to Canada have soared 181 percent. Imports from Canada have increased slightly less, 170 percent.

Last night, Republican Senator Susan Collins of Maine came to Canada's defense, tweeting, "We have had differences with Canada over the years. Nevertheless, Canada remains our close ally, good friend, and one of America's biggest trading partners."

Democratic Senator Dianne Feinstein of California said that the G7 agreement is more than symbolic.