The parties aiming to unseat the ruling conservatives in Mexico's July 2012 election have arrayed themselves in two rival coalitions.

The centrist Commitment to Mexico front is led by the Institutional Revolutionary Party, or PRI, in partnership with the Greens and the New Alliance party.

Spearheading the Progressive Movement is the Party of the Democratic Revolution, known as the PRD, joined by the Labor Party and the Citizens Movement.

In each case, the junior partners will back the presidential nominees of the lead party, while the coalitions hope to present joint candidates in many congressional races.

The PRI held the Mexican presidency from 1929 until 2000, when the conservative PAN's Vicente Fox won by a comfortable margin with support from across the political spectrum.

But the PAN's fortunes have suffered under Fox's successor, Felipe Calderon, who has presided over Mexico's worst economic slump since the Depression and waged a war against drug cartels that has cost nearly 50,000 lives.

The PRI, which made big gains in the 2009 mid-term congressional ballot, is heavily favored to retake the highest office in next year's elections.

"I celebrate that we could arrive at this grand accord," PRI chairman Humberto Moreira said after announcing the electoral pact.

He told reporters the centrist alliance's main rival in 2012 will be former Mexico City Mayor Andres Manuel Lopez Obrador, who emerged this week as the PRD's standard-bearer.

Lopez Obrador lost the 2006 presidential contest by 0.5 percent of the vote to Calderon.

Claiming fraud, Lopez Obrador demanded a full recount, but election authorities agreed to review only a small percentage of the ballots and the top electoral court proclaimed Calderon the winner, even though judges acknowledged that PAN incumbent Fox's efforts on behalf of his party colleague had violated the law.

Lopez Obrador's supporters occupied parts of downtown Mexico City for months after the election and the candidate subsequently held a massive rally in the capital where he declared himself the country's "legitimate president."

The PRI's presidential nominee is almost certain to be the telegenic former governor of Mexico state, Enrique Peña Nieto, who currently enjoys a wide lead in the polls over Lopez Obrador.