By Dave Graham
MEXICO CITY (Reuters) - Mexico's main leftist party will weigh tying up with conservative rivals to wrest power in 2018, after their successful alliance handed President Enrique Pena Nieto a beating in state elections, its leader said on Wednesday.
On Sunday, the Party of the Democratic Revolution, or PRD, teamed up with the center-right National Action Party (PAN) to inflict one of the worst ever electoral defeats on Pena Nieto's Institutional Revolutionary Party (PRI).
The 12 contests took place in the most fragmented political landscape in modern Mexican history and PRD Chairman Agustin Basave said in an interview no party would be able to win the next presidential elections without doubling up.
"I don't rule out any potential alliance," he said. "What I rule out is going it alone."
For 71 consecutive years the PRI ruled Mexico via a mix of political patronage, corruption and authoritarianism until it was voted out in 2000. In 2012, the party retook the top job under Pena Nieto and will try to hang onto the post in Mexico's 2018 general election.
While sidelined, the PRI still controlled at least half the 31 federal states, and since returning to power, adversaries say it has exploited its advantage to keep the opposition divided.
Basave, who like many other Mexican political leaders once belonged to the PRI, said if the party managed to split the opposition vote in 2018 it could field a "horse" as candidate and still come out on top.
"That's why I say that if you're against alliances in Mexico today, you're pro-PRI," he said.
The PRI started Sunday with 9 of the 12 states but ended up with only five. Three of those losses went to PRD-PAN coalitions while PRD leaders also backed the PAN contender in a fourth.
The PRD's base has been eroded by a new left-wing party founded by its former leader Andres Manuel Lopez Obrador. Now a frequent critic of his old party, he accuses the PRD of selling out and has dismissed talk of forging alliances with it.
Basave said he was open to working with both Lopez Obrador or the PAN, but noted that as long as the former remained opposed to the idea, it was "(Lopez Obrador's) problem".
"I'm not saying the PRD has to form an alliance with the PAN, we'll have to think, wait and see," he said.
(Reporting by Dave Graham; Editing by Simon Gardner and Andrew Hay)