MANILA, Philippines ? The upcoming merger of two administration parties is a threat to the opposition and will serve as a ?challenge? for it to consolidate its ranks, a spokesman for the United Opposition said.

?We have no illusions it will be easy but if the opposition can agree that if we do unite, then we can get the best scenario,? lawyer Adel Tamano said during the weekly Fernandina Forum on Wednesday.

The Kabalikat ng Malayang Pilipino (Kampi), the party founded by President Gloria Macapagal-Arroyo, will seal its merger with the Lakas Christian Muslim Democrats on Thursday.

Many members of the opposition personalities have expressed interest in running for president in 2010, including Senators Panfilo Lacson, Manuel Roxas II, Francis Escudero, and Loren Legarda, and Makati Mayor Jejomar Binay, who is also UNO president.

Former president Joseph Estrada had said that he would seek the presidency again if the opposition failed to field a sole candidate. But this was questioned by his critics, citing provisions in the Constitution that a president could not seek ?any reelection.?

Tamano said Estrada running again for president remained as the opposition's best option because of the former leader?s mass appeal.

Estrada, who was ousted by a military-backed popular revolt in January 2001, was convicted of plunder in September 2007, but was pardoned by Arroyo a month later.

Arroyo was Estrada?s former vice president, who took over the presidency after the latter?s fall from power.