Updated

President Bush, replying to criticism from Democratic presidential candidates, said Tuesday that "now is not the time to renegotiate ... or walk away from" the North American Free Trade Agreement.

"Now is the time to make it work better for all our people, and now is the time to reduce trade barriers worldwide," Bush said.

He also used a joint news conference with Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper to renew his call for Congress to pass a free-trade pact with Colombia. He said House Speaker Nancy Pelosi's refusal to schedule a vote could effectively kill it this year.

"It makes no sense to me to say that Colombian goods can come into our country duty-free, yet our goods can't go into Colombia duty-free," Bush said sternly. "And yet that's the case. An agreement with Colombia would level the playing field."

NAFTA has been criticized by both Democratic Sens. Hillary Rodham Clinton and Barack Obama, who have promised to try to renegotiate the agreement if elected president. Bush's comments came on the day of the critical Pennsylvania primary, in which trade — and the loss of manufacturing jobs in the United States — has been a top issue.

Bush was asked if he thought he was losing the free-trade debate in the court of public opinion because of the strong opposition to NAFTA from the two Democratic rivals.

"My biggest concern on trade is with Colombia. NAFTA exists," Bush said, saying the pact was mutually beneficial to the United States as well both of its North American neighbors.

"Now is not the time to renegotiate NAFTA or walk away from NAFTA," Bush said. The agreement, which took effect in 1994, was pushed by Clinton's husband, President Clinton.

Calderon echoed Bush's comments on standing by the agreement.

"We agreed that this is not the time to even think about amending it or canceling it. This is the time to strengthen and reinvigorate it," the Mexican president said.

"Jobs have grown in all three" countries as a result of NAFTA, Calderon said. "The benefits are visible." He declined to comment on statements on NAFTA by either the two Democratic contenders or by presumptive GOP nominee John McCain, who supports NAFTA as well as pending free-trade agreements.

Still, layoffs in U.S. manufacturing and the current economic slowdown have made NAFTA and other free-trade deals the subject of intense criticism in the Democratic-led Congress.

"I'm concerned about protectionism in America. It is not in our interest to become a protectionist nation," Bush said.