The White House on Friday vigorously defended the 14-year-old free-trade agreement among the United States, Mexico and Canada against sharp criticism from Democratic presidential candidates Hillary Rodham Clinton and Barack Obama.

"There's nothing broken. Why fix it?" said Dan Fisk, senior director of Western hemisphere affairs for National Security Council. He acknowledged the administration must do a better job of explaining the benefits of the agreement.

Both Clinton and Obama have threatened to pull out of the North American Free Trade Agreement to pressure Canada and Mexico to negotiate more protections for workers and the environment in the agreement. The accord has removed most barriers to trade and investment among the three countries.

NAFTA will be a major topic when President Bush joins Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon in New Orleans on Monday and Tuesday for his fourth and final North American Leaders' Summit.

Bush has suffered a major setback on the trade front with the derailing of a proposed free-trade pact with Colombia. Bush sent the agreement to Capitol Hill earlier this month, but the House, led by Democrats, decided to eliminate a rule forcing a vote on the deal within 60 legislative days. The House's decision probably kills consideration of the agreement this year, leaving it for the next administration.

"Leaders in Congress have made a serious error," Bush said in a speech. "A serious error for economics reasons. A serious error for security reasons. It's not too late, however, for them to get it right."

On NAFTA, the White House contends the agreement has been a boon for the economies of all three countries. Three-way trade has swelled from roughly $290 billion in 1994 to about $1 trillion by the end of this year, Fisk said at a briefing on next week's summit.

"We think NAFTA works," he said. Fisk said the criticism from the campaign trail has gotten a lot of attention from U.S. trading partners. "Some of the statements that have been made here have made bigger headlines in Canada and Mexico than they have here," he said.

"We want to find ways to, frankly, convince the American people from our perspective first and foremost that this is an arrangement that has worked for us and it's also worked for our neighbors," he said. "It's been a win-win situation."