The second-term Brazilian president is a former trade union leader, while Evo Morales, Bolivia's president, is an indigenous Indian and Paraguay's president, Fernando Lugo, was formerly a Catholic bishop.
Alvaro Uribe, the president of Colombia, has also said he expects Obama to win the November 4 US presidential election.
At a public debate with Colombian trade union leaders, the staunch Bush administration-ally said he intended to talk to Obama "when he is president of the United States'' about the country's restrictions on unions.
Obama has cited violence against union members that has killed around 31 people this year, as grounds to oppose a free trade agreement with Colombia.
McCain, however, has backed the treaty, which is currently stalled in the US congress.