July 6 (Reuters) - A congressional election defeat for
Mexico's ruling conservatives has left President Felipe
Calderon's economic reform agenda in the hands of the
opposition, which might put together a coalition with an
absolute majority.

The opposition Institutional Revolutionary Party, or PRI,
scored a big victory and will become the largest force in the
lower house of Congress.

Calderon's National Action Party, or PAN, will fall into
second place, while the leftist Party of the Democratic
Revolution, or PRD, could drop into distant third. The PAN will
continue to be the largest single party in the Senate.

Official results could be days away, but some observers see
the PRI garnering enough seats to partner with the Green Party
and form a block with more than 250 of the 500 lower house
seats. PRI party chief Beatriz Paredes said on Sunday the party
might seek to form that block.

Following is a possible breakdown of the lower house by
party, consolidating the projected ranges seen by polling firm
Mitofsky and by Reforma newspaper:

Party Current seats held Projected seats held

PRI 106 237-245

PAN 206 138-143

PRD 126 72-76

Green Party 17 17-21

Labor Party 11 11-13

New Alliance 9 5-8

Convergence Party 18 4-6

(Reporting by Jason Lange)