Mexicans will vote in national elections on Sunday with the ruling party candidate, Claudia Sheinbaum, commanding a hefty lead in the polls and expected to become the country's first female president.

Sheinbaum's mentor and popular outgoing president, Andres Manuel Lopez Obrador, has loomed over the campaign, seeking to turn the vote into a referendum on his political project that Sheinbaum, a leftist, has vowed to continue.

"We already won the campaign, we already won the debates and now we have to win the election to consolidate the continuity of the national project," 61-year-old Sheinbaum, a trained physicist and former Mexico City mayor, said as she wound up her campaign.

Sunday's elections are the biggest in Mexico's history, with voters electing about 20,000 posts. 

The contest has been marred by violence with 37 candidates murdered during the campaign, the most in the country's modern history, stoking concerns about the threat of warring drug cartels to Mexico's democracy.

Polls have consistently placed Sheinbaum about 20 percentage points ahead of her closest challenger, Xochitl Galvez, a businesswoman and senator who heads an opposition coalition comprised of the Institutional Revolutionary Party (PRI), which ruled Mexico for about seven decades until democratic elections in 2000, the right-wing PAN, and the leftist PRD party.

Either women's victory will be heralded as a major step in Mexico, becoming the first female leader in a country often criticized for its macho culture.

The winner will face formidable challenges, especially how to tame organized crime violence that contributed to more than 185,000 people being murdered since Lopez Obrador took office in Dec. 2018.

That violence, along with electricity and water shortages, is a problem as Mexico attempts to persuade manufacturers to relocate as part of the nearshoring trend, in which companies move supply chains closer to their main markets.

The winner will also have to wrestle with what to do with Pemex, the state oil giant which has seen production decline for two decades and is drowning in debt.

Both candidates have promised to expand welfare programs, which could be a challenge amid a large deficit this year and sluggish GDP growth of just 1.5% expected by the central bank next year.

Sheinbaum has rejected opposition claims that she would be a "puppet" of Lopez Obrador though she has pledged to continue many of his policies including those that have helped Mexico's poorest.

"Our president really cares about poor people," said Alejandro Benitez, 68, who plans to vote for Sheinbaum despite living in Tepatepec, a town in the Hidalgo state where her opponent Galvez grew up.

The new president, who is set to begin a six-year term on Oct. 1, will also face a series of tense negotiations with the United States over the huge flows of U.S.-bound migrants crossing Mexico and security cooperation over drug trafficking at a time when the U.S. fentanyl epidemic rages.

Mexican officials expect these negotiations to be more difficult if the U.S. presidency is won by Donald Trump in November. 

Trump, the first U.S. president to be convicted of a crime, has vowed to slap 100% tariffs on Chinese cars made in Mexico and said he would mobilize special forces to fight the cartels, a hot button issue in a country which lost vast territory to a U.S. invasion in the 19th century.

Almost 100 million Mexicans are eligible to vote in Sunday's election, where key positions up for grabs include the capital city's mayor, eight governorships, both chambers of Congress, and a slew of regional and local posts.

Polls indicate Morena is likely to fall short of a two-thirds majority in Congress, which would have allowed Sheinbaum's party to approve constitutional reforms that eluded her predecessor.

The polls open at 8:00 a.m. local time (1400 GMT) and will close at 6:00 p.m. local time (0000 GMT on Monday). The first official preliminary results are expected late tonight.