When President Mahmoud Ahmadinejad of Iran begins a four-nation tour of Latin America on Sunday, showcasing his support in the region against a backdrop of international tensions over his nation’s nuclear program, he is set to visit some of the United States’ most ardent critics: Venezuela, Nicaragua, Cuba and Ecuador.

But the list of countries Mr. Ahmadinejad will not be visiting is equally telling.

Though Iran is reeling from successive rounds of international and unilateral sanctions, Mr. Ahmadinejad is not visiting Brazil, the region’s economic powerhouse. Nor is he going to other large countries like Mexico, Colombia and Argentina, underscoring that his visit is limited to nations without extensive influence or the capacity to offer much of a major economic partnership.

“If Iran is intent on spreading its political influence, it will not find a hospitable environment in Latin America,” said Michael Shifter, president of the Inter-American Dialogue. “Most governments may want more breathing space from Washington, but they are not looking to be aligned strategically with Tehran.”

The trip, Mr. Ahmadinejad’s sixth official visit to the region since becoming president in 2005, seems intended as a counterstrike against Iran’s increasing international isolation. But his previous trips have not always gone smoothly; his one visit to Brazil, in 2009, was met with protests.