The February survey by polling firm Parametria showed Sheinbaum, from the National Regeneration Movement (MORENA) with 49% support, ahead of Xochitl Galvez, the candidate for an opposition alliance of three parties, with 29%.
A prior October-November survey by the firm showed Sheinbaum, Mexico City's former mayor, at 52% support, and businesswoman-turned-politician Galvez at 25%.
A win for either Sheinbaum or Galvez would give Mexico its first female president.
The poll gave third-party contender Jorge Alvarez of the opposition center-left Citizens' Movement (MC) backing of 5 percent, while showing 17% of respondents offered no preference.
Parametria's survey polled 800 people between Feb. 7 and 11, with a margin of error of 3.5 percentage points.
With the backing of Mexico's popular president, Andres Manuel Lopez Obrador, who has consistently polled support of around 60% or more, Sheinbaum has been favored as the next in line for the seat.
Of poll respondents, 78% had a positive opinion of Sheinbaum, compared with 42% for Galvez.
Mexican law limits presidents to a single six-year term.
Whoever wins the race will likely face pressure to address the widespread presence of criminal groups across the country, reduce associated violence and pursue policies that enable Mexico to benefit from a trend among businesses to move industrial supply chains to North America from Asia, known as nearshoring.
(Reporting by Sarah Kinosian; editing by Jonathan Oatis)
