MEXICO CITY -- Republican presidential candidate John McCain said yesterday that a shake-up in the leadership of his campaign was part of a "natural evolution" as the organization becomes more national in scope.

McCain's campaign announced Wednesday that top adviser Steve Schmidt would assume a broad portfolio of duties, with nearly full control over message and strategy. Schmidt will report to Rick Davis, who will keep the title of campaign manager but focus on longer-term matters such as the Republican National Convention and McCain's running mate.

Addressing reporters at the conclusion of a three-day visit to Colombia and Mexico, McCain downplayed the personnel shift.

"Our campaign continues to grow, and the responsibilities are expanding, and Mr. Schmidt is taking over some increased responsibilities," McCain said. "Rick Davis remains the campaign chairman, campaign manager. It's a natural evolution . . . with increased staff and increased responsibilities."

The staff shake-up threatened to overshadow McCain's trip through Latin America and his free-trade message.

McCain met yesterday with Mexican President Felipe Calderon to discuss trade and immigration issues. They spoke at length about the Merida Initiative, a U.S.-sponsored effort to stem the flow of drugs and guns across the Mexican border, McCain said.

McCain began the day at the Basilica de Guadalupe, Mexico's holiest Roman Catholic site, where he received a blessing from the Basilica's monsignor.

Catholic and Hispanic voters are expected to be key in the November election.

McCain's trip to Colombia and Mexico was billed primarily as an opportunity to promote free trade in the Western Hemisphere.

Obama has spoken out against NAFTA, the North American Free Trade Agreement, and a pending free-trade pact with Colombia, both of which are unpopular in important general election swing states such as Ohio. McCain wants to help workers displaced by free-trade agreements receive job training and other benefits.