CARTAGENA, Colombia—John McCain portrayed free trade Tuesday as a win-win proposition for the U.S. and its Latin American economic partners, but labor leaders said it’s been a big loser for Rust Belt voters.

The Republican presidential hopeful began a three-day visit to Colombia and Mexico after a campaign swing through Indiana and Pennsylvania, two states hard hit by the loss of manufacturing jobs due in part to trade agreements like the North American Free Trade Agreement between the U.S., Mexico and Canada, which McCain strongly supports.

As McCain headed abroad, Democrat Barack Obama repeated his vow to renegotiate NAFTA if elected to include enforceable labor and environmental provisions. At a news conference in Ohio, a state hard-hit by job losses, Obama said, “The United States wanting to make sure that its ... standards aren’t being undermined isn’t imperialist.”

Although Obama didn’t mention McCain or his trip, a prominent Obama supporter criticized McCain’s visit on a conference call with reporters.

“Today after he finishes his speech here in Indiana, he’s hoping on a plane and going to Colombia and Mexico to talk about how much our trade agreements are going to help those countries, rather than taking about what we can do to help this country,” United Auto Workers vice president Terry Thurman said.

“Now I find it no surprise that he’s going to go to Mexico to talk about how great NAFTA is because he’s certainly is not going to find much support for it here in the Hoosier state.”

McCain conceded Monday he still has work to do to convince voters in industrial swing states in the Midwest, where the presidential election could be decided, that his support for free trade will benefit them, not just cost more jobs. He pledged to improve programs for displaced workers and unemployment insurance if elected, but acknowledged that wouldn’t be enough.

“I have to convince them the consequences of protectionism and isolationism could be damaging to their future,” the Arizona senator said.

“I understand it’s very tough. But for me to give up my advocacy of free trade would be a betrayal of trust,” he said. “And the most precious commodity I have with the American people is that they trust me.”

McCain’s trip caps a visit in late June to Ottawa, where he talked up cross-border cooperation with Canada on economic issues, especially trade.

When McCain went to Canada, Obama suggested he went to promote the agreement rather than stay home and defend his views in places like Ohio and other states where NAFTA is blamed for shifting millions of manufacturing jobs to other countries.

NAFTA and other free trade deals were a flash point in Obama’s primary campaign against Hillary Rodham Clinton.

McCain insisted the trip to Latin America was not intended to be political and said he would not criticize Obama directly while abroad.

He made a similar pledge when he visited Canada but took a swipe at the Democrat nonetheless.