Mexico City, Jun 5 (IANS): Mexico is gearing up for the largest ever mid-term elections in the country's history on Sunday, with tens of thousands of candidates vying to fill more than 20,000 public posts, according to authorities.

The race to watch, said political observers, is the one to fill the Chamber of Deputies, or the lower house, in which the ruling progressive National Regeneration Movement (Morena) hopes to maintain or strengthen its qualified majority, as the conservative opposition strives to gain ground, reports Xinhua news agency.

The mid-terms will essentially decide whether President Andres Manuel Lopez Obrador will be able to carry on with his so-called "Fourth Transformation", combat public-sector corruption and make the country more energy independent, or be forced to temper his reform drive during his remaining time in office.

Up for renewal are 15 of the country's 32 Governor posts, 500 federal deputy seats, 1,063 local deputy seats and 1,923 mayoral posts, among others, with a total of the 20,415 popularly elected positions.

According to the National Electoral Institute, 93,676,029 Mexicans will be eligible to vote at 162,896 polling stations across the country.

To improve their chances in the lower house, Mexico's three largest opposition parties -- the Institutional Revolutionary Party, Democratic Revolution Party and National Action Party -- joined forces to back the same candidate for each seat, aiming to take votes away from the Morena-led coalition that includes the Labor Party and Ecologist Green Party of Mexico.

To maintain its two-thirds qualified majority in Congress, Morena will need to garner 334 deputy seats, a number which Lopez said will be tough to win, despite the party's lead in various polls.

The elections are expected to go smoothly, though violence targeting candidates has traditionally marred political campaigns in Mexico with some 89 politicians assassinated so far, including 35 candidates, according to risk analysis firm Etellekt.