MEXICO CITY, June 6 (Xinhua) -- Mexico's ruling Institutional Revolutionary Party lost seven of 12 gubernatorial races to the opposition in Sunday's regional elections, which is expected to help set the tone for the next presidential election in 2018.

Based on preliminary vote counts on Monday, the conservative National Action Party is the big victor in the gubernatorial races, leading in seven states. In three of these contests, it fielded a candidate in alliance with the Party of the Democratic Revolution.

The losses of the Institutional Revolutionary Party include two oil-rich strongholds in the Gulf Coast state of Mexico, Veracruz and neighboring Tamaulipas, as well as Quintana Roo, home to Mexico's top tourist destination Cancun.

"For the first time in 86 years, Veracruz will not be governed by the Institutional Revolutionary Party," said news website Animal Politico.

With the result far worse than most polls had forecast, the party's Secretary General Carolina Monroy del Mazo said in a interview, "Not even in our worst-case scenario did we expect to lose seven governorships."

The strong showing of the National Action Party has raised its hopes for regaining the presidency in 2018, while underscoring the country's deep dissatisfaction with Mexican President Enrique Pena Nieto and his ruling Institutional Revolutionary Party.

However, Nieto stressed the democratic process of the elections, saying "no one can deny that in Mexico it is the people who elect their representatives, because their votes count."

The elections were also a test for Mexico's fledgling left-leaning National Regeneration Movement, a party founded two years ago by former presidential candidate and Mexico City mayor Andres Manuel Lopez Obrador, who plans to run again in 2018.

Candidates of the National Regeneration Movement lost all the gubernatorial races, but the party did well in the capital, where voters were called to elect 60 members of a new constitutional assembly to draft the city's first ever constitution.

As the seat of the federal government, Mexico City was officially called the Federal District until earlier this year, when that designation was dropped.

In a bid to grant the city state-like status, a new constitutional assembly is to draw up a constitution by the end of the year.