Leftist front-runner Andres Manuel Lopez Obrador of the National Regeneration Movement (MORENA) speaks during his campaign rally in Cuautitlan Izcalli, Mexico, April 13, 2018. REUTERS/Henry Romero

MEXICO CITY (Reuters) - Mexican leftist Andres Manuel Lopez Obrador has pulled further ahead of his presidential rivals in the run-up to a July 1 election, a voter survey by polling firm Mitofsky showed late on Wednesday.

Former Mexico City mayor Lopez Obrador received 31.9 percent support in the poll conducted from April 13 to 15, up from 29.5 percent in a previous Mitofsky survey last month.

Running second was Ricardo Anaya, a former chairman of the center-right National Action Party, who heads a right-left alliance. His support declined for a second straight month, slipping to 20.8 percent from 21.2 percent the prior poll.

Jose Antonio Meade, candidate of the ruling Institutional Revolutionary Party (PRI), remained in third place, gaining slightly to 16.9 percent from 16.4 percent, the poll showed.

Some 25 percent of those surveyed said they had no voter preference.

The Mitofsky poll surveyed 1,000 registered voters in face-to-face interviews, and had a margin of error of 3.1 percentage points.