(For a TAKE A LOOK on Mexico, click [nN25302940])

* Calderon's party likely to lose ground

* Defeat may hold back key oil and tax reforms

* Former ruling PRI party set to win
(Adds quotes from voters)

By Alistair Bell

MEXICO CITY, July 5 (Reuters) - Mexicans voted on Sunday in
mid-term elections for Congress where President Felipe
Calderon's party is likely to lose ground, leaving him with an
uphill struggle to achieve economic reforms.

Mexico's economy is in deep recession, mostly due to the
downturn in the United States, and oil output is falling fast.

Calderon, a conservative, wants to overhaul the energy
sector to allow more private investment in the search for oil
in the Gulf of Mexico. He also seeks to reform the tax system.

But his National Action Party, or PAN, trails the
opposition Institutional Revolutionary Party, or PRI, by around
5 percentage points in opinion polls.

Even if the PAN wins, Calderon will probably fall short of
the majority he needs to push reforms through the lower house
of Congress without long, difficult negotiations.

Calderon is personally popular but the economy is likely to
weigh against his party, which promised change in 2000 when it
ended 71 years of one-party rule under the PRI.

"Things definitely were not as bad under the PRI as they
are for us now," said Pamela Gonzalez, 25, who is studying
business administration in the capital.

The election should not hurt Calderon's war against drug
cartels. More than 12,300 people have died since Calderon
dispatched the army to battle drug gangs in 2006, but Congress
has not played a major role in the fight.

NEEDS OPPOSITION BACKING

Calderon has lacked a majority in Congress since taking
office in 2006 but more than ever he needs backing from
opposition deputies to approve reforms.

Mexico's tax take is one of the lowest in Latin America and
foreign investors want to see a fiscal overhaul. Decades of
sluggish economic growth have kept most Mexicans poor and
spurred millions to cross the U.S. border in search of work.

Mexican oil output has dropped to its lowest in 16 years,
eroding a pillar of public finance, and the economy is set to
shrink 6 percent or more this year due to the recession.

"Investment and exports are falling. It's a mess and it
will be a long time before we recover," university professor
Enrique Serrano, 51, said as he left a Mexico City polling
station.

The centrist PRI will likely take first place in the lower
house, doubling its number of seats to more than 210,
leapfrogging the struggling left-wing opposition, and leaving
the PAN in second place with 175 seats or less.

If the vote is close, the two parties are unlikely to
produce major reforms in the next three years, said Allyson
Benton of the Eurasia Group.

"The parties would likely continue to cooperate after the
elections, even though the extent of this cooperation will be
limited to producing only minor economic policy reforms," she
said.

The PRI might be more willing to help Calderon with reforms
if it wins big on Sunday, so as to get Mexico's economic house
in order ahead of the 2012 presidential election.

All 500 seats in the lower house of Congress are on the
table in Sunday's vote. Voters will also elect six state
governors and hundreds of mayors.
(Additional reporting by Michael O'Boyle; Editing by John
O'Callaghan)