UPDATE 6-As Mexicans vote, comeback beckons for old rulers

* Pena Nieto, ex-governor of State of Mexico, likely to win

* Financial investors encouraged by his reform agenda

* Apparent drug violence hits Acapulco on eve of vote

(Updates with fresh quote, early estimates for states)

By Lizbeth Diaz and Dave Graham

MEXICO CITY, Mexico, July 1 (Reuters) - Mexico's old rulers
were on track for a comeback as voters chose a new president on
Sunday, after a grisly war with drug cartels and a sluggish
economy wore down the ruling conservatives.

Twelve years after the Institutional Revolutionary Party
(PRI) lost power, opinion polls showed its candidate, Enrique
Pena Nieto, heading into the vote with a double-digit lead over
his opponents.

Voters ousted the PRI in 2000 after 71 years of virtual
single-party rule that was tainted by corruption, electoral
fraud and authoritarianism.

But Pena Nieto has established himself as the new face of
the party, which has bounced back, in part because of economic
malaise and lawlessness under the conservative National Action
Party (PAN).

A noisy crowd of protesters met Pena Nieto when he voted in
Atlacomulco, about two hours northwest of the capital, but
hundreds of his supporters shouted down the demonstrators.

A youthful-looking former governor of the State of Mexico,
Pena Nieto promises reforms to improve the country's tax take,
loosen the job market and open the state-owned oil firm Pemex to
more foreign investment, citing Brazil's Petrobras as a model.

Mexicans are fiercely protective of Pemex, but the PRI,
which nationalized oil production in 1938, could be the one
party able to liberalize the energy sector.

"It's time for the PRI to return. They're the only ones who
know how to govern," said Candelaria Puc, 70, preparing to vote
in the beach resort of Cancun with the help of a friend because
she cannot read or write.

"The PRI is tough, but they won't let the drug violence get
out of control," she said in a mix of Mayan and Spanish.

Others feared a return to the worst years of PRI rule and
put Pena Nieto's big lead down to his cozy relationship with
Televisa, Mexico's top broadcaster.

"It's the same party as ever and the people who vote for him
(Pena Nieto) believe they are going to live happily ever after
like in the soap operas," Humberto Parra, a systems engineer,
said as he went to vote in Mexico City.

Pena Nieto's closest challenger in pre-election polling was
Andres Manuel Lopez Obrador, a leftist former Mexico City mayor
often referred to by his initials AMLO, who narrowly lost the
2006 election to President Felipe Calderon of the PAN.

Exit polls from several regional elections also being held
on Sunday showed the PRI was likely to capture the major western
state of Jalisco from the PAN, and that the leftist Party of the
Democratic Revolution (PRD) had kept control of Mexico City.

The PRI's allies, the Greens, were seen picking up the
southern state of Chiapas from the PRD.

VIOLENCE HITS CONSERVATIVES

Lopez Obrador claimed fraud after the 2006 election and
launched months of street protests that failed to overturn the
result and instead alienated many former supporters. His claims
that the PRI is this time planning fraud have raised concerns of
more protests, although polls suggest Lopez Obrador will fall
short of the 35 percent of votes he won in 2006.

"This is no time for the country to go in reverse," a
relaxed Lopez Obrador said of the PRI before voting.

An official for the Federal Electoral Institute (IFE) said
the election had probably registered the lowest number of
complaints in Mexico's history.

But Lopez Obrador supporters stood ready to hit the streets
if need be.

"We are with AMLO. I'm sure there will be fraud, and we will
be ready to back him up if they steal the presidency," said
Beatriz Sosa, 30, playing soccer with her six-year-old son in a
park in the capital's upmarket Polanco neighborhood.

Bidding to become the country's first female president, PAN
candidate Josefina Vazquez Mota was third in pre-election polls.

The PAN ended the PRI's long rule in 2000 but years of weak
growth and the death of more than 55,000 people in drug-related
killings since 2007 have steadily eroded its popularity.

Violence continued in the days before Sunday's vote.

In the Pacific beach resort of Acapulco, one of the cities
most affected by the drug war, four people were killed on
Saturday, two of them tortured and beheaded, a hallmark of
drug-related killings.

The PRI mayoral candidate in the city of Marquelia, down the
coast from Acapulco, was kidnapped for more than seven hours by
an armed group, but released early on Sunday.

Final polls showed Pena Nieto winning 40 percent to 45
percent of the vote and Lopez Obrador close to 30 percent with
Vazquez Mota not far behind. The candidate with the most votes
wins, with no need for a second round.

The first national exit polls were expected when voting ends
in the westernmost part of the country at 8 p.m. Mexico City
time (2100 EDT/0100 GMT).

The PRI laid the foundations of the modern state with a
nimble blend of politics and patronage that allowed it to appeal
to labor unions and captains of industry at the same time.

Mexicans eventually tired of the one-party rule that stifled
dissent, rewarded loyalists and allowed widespread corruption.

The era of old-time PRI bosses known as "dinosaurs" gave way
to a more democratic era under the 1994-2000 presidency of
Ernesto Zedillo, who instituted reforms that allowed opposition
parties to compete in a fair vote and oust the PRI.

Voters also elect both houses of Congress on Sunday.

The legislative results will help determine whether Pena
Nieto will be able to push through his reform agenda.

(Additional reporting by Dave Graham, Ioan Grillo, Gabriel
Stargardter, Mica Rosenberg, Tomas Sarmiento, David Alire
Garcia, Simon Gardner, Isela Serrano, Michael O'Boyle and Luis
Enrique Martinez; Editing by Christophr Wilson and Daniel
Trotta)