Mexican opposition leader Andres Manuel Lopez Obrador, who twice contested second-place losses in presidential elections, said on Sunday he would leave his coalition, a move that threatens to create a rift among leftists in Congress.

"I have separated from the parties that form the Progressive Movement," Lopez Obrador said. "This isn't a rupture, I leave in the best of terms."

Lopez Obrador said he would dedicate all his efforts to change Mexico with a new organization called Morena that has yet to be legally incorporated as a party.

Lopez Obrador accused President-elect Enrique Pena Nieto of laundering money and buying votes in July's presidential election, but he lost a legal bid at the end of August to overturn the results.

Pena Nieto will be sworn in December 1 and has promised fiscal, labor and energy reforms, which Lopez Obrador is likely to resist. Lopez Obrador's supporters blocked many of Mexico City's main thoroughfares for weeks after he narrowly lost the 2006 election. There were a few protests this year, but they failed to reach the scale of 2006.

Lopez Obrador called for a "peaceful civil resistance" before thousands of his supporters in the capital's main square, but he once again refused to recognize Pena Nieto as Mexico's legitimate president.

Javier Oliva, a political scientist at the National Autonomous University of Mexico, said as many as 15 deputies in the lower house and two or three senators from the leftist coalition will likely join Lopez Obrador in his new movement.

"This is going to have important repercussions," Oliva said. "This will create divisions and a future rupture among the left."