AGUASCALIENTES, Mexico (CNN) -- Voters in three Mexican states will go to the polls Sunday to pick new governors -- a key test of the strength for the country's three main political parities going into the 2000 presidential election.

The Institutional Revolutionary Party, or PRI, which has ruled Mexico for 69 years, suffered a serious blow in national elections last year, losing control of the lower house of Congress.

But PRI's fortunes have improved since then, and its candidates are favored in two of the three gubernatorial races, with a shot at also winning the third.

The most competitive of the three races is in the central state of Aguascalientes, where PRI's Hector Hugo Olivares Ventura, a former senator and federal deputy, is in a tight race with businessman Felipe Gonzalez Gonzalez of the center-right National Action Party, or PAN.

Alfonso Bernal Sahagun, of the center-left Democratic Revolution Party, or PRD, is a distant third.

In Veracruz, on the Gulf of Mexico coast, polls have shown that PRI's Miguel Aleman, a wealthy businessman and son of a former Mexican president, has a large lead over his nearest competitor, PAN's Luis Pazos, an economist.

The race will be closely watched to see how strong Aleman runs. He is considered a possible contender for the presidency in 2000.

In rural Oaxaca, one of Mexico's poorest states, PRI Sen. Jose Murat Casab is expected to hold off a PRD challenger for the governor's post.

In Mexico, gubernatorial races are particularly important because they often produce powerful political figures who can deliver crucial votes for their parties in national elections.

Sunday's ballot may be most crucial to the PAN, which is trying to bounce back from a string of defeats.

Last year, its candidate finished a distant third in the Mexico City mayoral race. And last month, it lost one of the six governorships it held, in the northern state of Chihuahua, to the PRI.