The Supreme Court is currently reviewing three petitions filed by several lawyers seeking Poe’s disqualification on citizenship grounds.
The movie star-turned-politician is President Arroyo’s closest rival in the May polls despite his lack of public office experience.
"In case of a disqualification, there should be a consolidated opposition to rally behind one candidate," Lacson said.
He accused the administration of orchestrating the petitions to derail Poe’s presidential run. "We know who is behind these petitions. That’s why the opposition should unite behind one candidate if he is disqualified."
Last month, Lacson and Poe’s camps tried to forge a united front and present a single opposition candidate against Mrs. Arroyo to avoid splitting the anti-Arroyo vote.
But merger talks failed after both sides refused to back down from their presidential bids although both agreed to submit themselves to a selection process.
Hopes for opposition unity were further dashed as opposition parties disagreed on whether to field Lacson or Poe as the standard-bearer.
Lawyers who filed the disqualification petitions claim Poe is not a natural-born citizen because he was born to parents who were foreigners.
Poe was born out of wedlock and therefore should have assumed his mother’s American citizenship. Children born out of wedlock should assume the citizenship of the mother, they said, citing the law prevailing at the time.
Poe’s lawyers argue that he was a natural-born Filipino because his father was a Filipino citizen despite his Spanish heritage.
Citing the 1935 Constitution, they contend that a person’s citizenship back then did not depend on whether he was born in or out of wedlock.
Members of Poe’s camp had warned of civil unrest if the popular movie actor is disqualified from the presidential race.
But the political neophyte dispelled fears of violent protests, saying he would not contest the Supreme Court if it disqualified him.