At a meeting in Venezuela earlier this month, Paraguayan President Nicanor Duarte brought up another source of division: alleged projectionist practices by Brazil and Argentina that have prompted Paraguay and Uruguay to question the benefits of Mercosur membership.

Chavez, however, called the display of leftist unity a "fiesta of integration. He lunched with Argentine President Nestor Kirchner in Buenos Aires before flying to Cordoba, arriving moments before Brazilian President Luiz Inacio Lula Da Silva.

Unlike Lula Da Silva, who boarded a black limousine without making comments, Chavez waded into a crowd of local journalists to speak of a new Mercosur, pledging to use Venezuela's oil wealth to bolster the once-sleepy customs union.

"We are entering a new stage of Mercosur," Chavez said. "Imagine that ... the incorporation of nearly 30 million Venezuelans into a southern common market, and the Venezuelan economy is one of the most vigorous today in the world."

The summit allows Chavez to trumpet one of his long-sought foreign policy goals weeks after some critics blamed his political meddling for contributing to the election of losses of leftist presidential candidates in Mexico and Peru. He has said Mercosur should be a front against U.S. free trade deals.

Venezuela's induction expands Mercosur beyond its beginnings in southernmost South America. It now includes all the continent's largest economies - Brazil, Argentina and Venezuela - along with Paraguay and Uruguay.

Morales and moderate leftist Chilean President Michelle Bachelet were attending as observers.

Castro, who turns 80 on Aug. 13, has traveled to fewer international summits in recent years. It was his first trip to Argentina since Kirshner's 2003 inauguration.

Cordoba holds special significance for Castro, a political mentor for the younger Chavez and Morales. The central Argentina province was the boyhood home of Ernesto "Che" Guevara, the Argentine who gave up a future in medicine to join Cuba's revolution.

The Mercosur leaders, meeting for the 30th time since 1991, are expected to sign a deal promoting trade between their nations and Cuba, which has been under a US embargo for decades.