FACTBOX: Mexico to elect new lower house, governors

(Reuters) - Mexico will elect a new lower house of Congress, six state governors and hundreds of mayors in mid-term elections on Sunday.

President Felipe Calderon's party is seen heading for defeat, hurting his ability to push economic reforms through Congress.

Here are some facts about the election:

CONGRESS

Voters will choose the 500 members of the Chamber of Deputies, the lower house. Three hundred are elected directly by voters. The other 200 are selected by parties, which win seats based on their share of the total vote.

The opposition Institutional Revolutionary Party, or PRI, is seen leaping into first place from third, replacing President Felipe Calderon's conservative National Action Party, or PAN, as the biggest force in the lower house.

Most opinion polls have shown the PRI ahead of the PAN in recent days by around 5 percentage points.

The centrist PRI currently holds 106 seats, while the PAN has 206. The center-left Party of the Democratic Revolution, or PRD, is seen coming in a distant third.

GOVERNORS

Voters will also elect governors in the states of Campeche, Colima, Queretaro, Nuevo Leon, San Luis Potosi and Sonora. Queretaro and San Luis Potosi are currently governed by the PAN and the rest by the PRI.

MECHANICS

Voting began at 8 a.m. (1300 GMT) and the last polling stations close 12 hours later in the far west of the country, which is two hours behind Mexico City.

The first official results from the Federal Electoral Institute will be released at 8 p.m. (0100 GMT on Monday). Electoral officials expect preliminary counts from about 40 percent of polling stations by midnight (0500 GMT on Monday).

TV exit polls for the congressional contests are expected at 8 p.m. (0100 GMT on Monday).

THE VOTERS

About 77 million people have the voting card credential needed to cast a ballot. Between 30 percent and 40 percent are expected to vote.

(Compiled by Jason Lange and Miguel Angel Gutierrez, Editing by Stacey Joyce)