MEXICO CITY — The man who led Mexico’s main leftist party in the past two presidential elections said Sunday he is leaving it and may start a new party, throwing uncertainty over the future of the nation’s political left.

Andres Manuel Lopez Obrador told supporters at a rally at Mexico City’s main plaza that he is leaving the Democratic Revolution Party ‘‘on the best of terms.’’ He also announced he is leaving the smaller Labor Party and Citizens’ Movement, which also backed him in the July presidential election, when he finished second.

Lopez Obrador said he will begin consultations that would create a party out of another, less formal organization that backed him, the Movement for National Regeneration.

The motives for the break were not clear, but it could complicate efforts for the left to rally again around a single candidate as it has in every election since 1988.

Lopez Obrador is the most prominent figure within the Democratic Revolution Party in recent years, one of only two people it has run for the presidency since forming after the fraud-tainted 1988 election.

Still, he has not been able to dominate the structure of the party, which has suffered through bitter internal feuds.

Lopez Obrador, who turns 59 in November, was one of many political figures who abandoned the ruling Institutional Revolutionary Party in 1988 to support the candidacy of Cuauhtemoc Cardenas.