In 1993, U.S. merchandise trade totaled $212 billion with Canada and $82 billion with Mexico, and the United States had an $11 billion trade deficit with Canada and a $2 billion surplus with Mexico.

Last year, U.S. merchandise trade totaled $562 billion with Canada and $347 billion with Mexico, and the United States had trade deficits of $64 billion with Canada and $74 billion with Mexico.

Although Mr. Obama acknowledged to Fortune magazine in June that his anti-NAFTA rhetoric during the primaries was "overheated and amplified," he still insists that NAFTA must be reopened. He has pledged to "amend" and "fix" it.

Mr. Obama voted against the Central American Free Trade Agreement, while Mr. McCain voted for it.

Mr. Obama voted to impose a 27.5 percent tariff on imports from China if Beijing failed to revalue its currency. Mr. McCain voted against it.

Mr. Obama has consistently voted for farm subsidies, which have been a major stumbling block in the multilateral trade-liberalization negotiations in the Doha development round. (The purpose of Doha, whose latest talks recently collapsed again, is to bring billions of people out of poverty by opening the markets of wealthy nations to the food exports of developing countries.)

Campaigning in Iowa last year, Mr. McCain opposed farm subsidies and ethanol mandates. He has repeatedly voted against farm subsidies.

Both Mr. Obama and Mr. McCain supported the Peru free-trade agreement. However, Mr. Obama opposes the Colombia free-trade agreement and the South Korea free-trade agreement, both of which Mr. McCain supports.

Despite Mr. Obama's current opposition, Gary Hufbauer, a senior fellow at the Peterson Institute for International Economics, said, "Obama, once in office, would probably pursue the South Korean and Colombian agreements because failure to do so would hurt U.S. credibility."

In recent years, the United States has racked up record trade deficits.

From 1988 through 1997, the U.S. trade deficit in goods and services averaged $84 billion per year. From 1998 to 2007, however, the annual trade deficit averaged $489 billion. And during the past three years, it has averaged $727 billion.

Manufacturing employment remained virtually unchanged during the first 10-year period (1988- 97), when manufacturing output, according to the Federal Reserve, increased 43 percent as manufacturing productivity (output per hour) jumped 38 percent.

During the second 10-year period, however, as trade deficits have soared, manufacturing employment has plunged by 22 percent, falling from 17.6 million jobs to 13.8 million. Nonetheless, manufacturing output has risen 22 percent during the past 10 years, in large part because productivity improved by nearly 50 percent.

"Through the first five months of this year, U.S. manufacturers enjoyed a trade surplus with our free-trade agreement (FTA) partners," said Mr. Huether, the NAM economist. "Contrary to the widely held view that our trade agreements are the cause of the U.S. trade deficit, FTAs are actually a bright part our current trade picture."

While manufacturing jobs have disappeared, the U.S. economy has added more than 25 million jobs since NAFTA was implemented, NAFTA supporters argue. More than half of those jobs were created during the past 10 years, when the trade deficit began to soar.

NAFTA opponents have countered that many of those jobs were created in lower-paying service sectors, which did not provide benefits comparable to those in the manufacturing sector.

"The overwhelming majority of jobs created since NAFTA have had nothing to do with the global economy," Mr. Tonelson said.

Meanwhile, the average U.S. unemployment rate declined from 6.6 percent during the 10 years (1984-93) before NAFTA became effective to 4.9 percent during the latest 10-year period (1998-2007). Since 1993, U.S. economic growth has averaged 3 percent per year, while the American economy has expanded by more than 50 percent.

"Since the mid-1990s," Mr. Tonelson argued, "U.S. economic growth has been supported by bubbles.