President Nicanor Duarte resigned Monday two months before his term ends, but opposition parties say they will fight the move by Duarte to claim a crucial seat in Paraguay's Senate.

Duarte walked to Congress to deliver his resignation in person, leaving Vice President Francisco Oviedo in charge until his term ends Aug. 15. Congress is due to consider the resignation on Tuesday.

Duarte says he wants to continue to serve his country from the Senate, to which he was elected in April. But opponents see it as a maneuver that would help Duarte extend his political power, avoid possible corruption charges and perhaps launch a return to the presidency.

Paraguayan presidents cannot serve consecutive five-year terms, so Duarte sought and won a Senate seat. To take office there, he must leave the presidency before July 1, when new senators must be sworn in.

President-elect Fernando Lugo, a former bishop who broke the 61-year rule of Duarte's Colorado Party, says he supports Duarte's right to resign. But he has not convinced most of his allies in Congress to do the same.

Congressman Armando Espinola, a leader of the Authentic Radical Party, said his faction would boycott Tuesday's meeting to prevent a quorum for the vote to approve Duarte's resignation.

That way, after Aug. 15, Duarte would lose the immunity from prosecution that public servants enjoy "and can be judged by justice if complaints of corruption are lodged against him," Espinola said.

While Lugo won the presidency, Duarte's party remains the largest in Congress and he would be in a strong position to lead the party's Senate delegation.