Enrile’s resignation triggered questions whether or not the opposition is having a hard time in filling up its prospective 12-man senatorial lineup as the Feb. 12 deadline for the filing of certificates of candidacy leading to the May 14 mid-term election approaches.

Estrada’s son, Sen. Jose "Jinggoy" Estrada, confirmed Enrile’s resignation, but stressed that Enrile is not leaving the party and would stay on as a member.

Enrile, a former justice and defense secretary during the Marcos administration, turns 83 on Feb. 14.

Reliable sources said Enrile wanted to give Estrada a free hand in selecting the opposition’s senatorial bets .

Enrile said he is not comfortable with some of the senatorial candidates so far selected by Estrada, sources added.

The opposition, initially headed by Estrada’s PMP and the United Opposition (UNO) of Makati Mayor Jejomar Binay, has "a revolt in its hands," analysts said.

Former Sen. Francisco S. Tatad questioned its apparent drift toward political dynasty and acceptance in its slate persons who were with the Arroyo administration but are now leaving it.

This prompted Tatad to leave the PMP-UNO senatorial slate being formed by Estrada, saying he could not endorse some of its candidates to the electorate.

Former senators Vicente " Vic"Sotto III and Teresa Aquino-Oreta, who had been attending meetings of Estrada and Binay on the formation of the opposition’s senatorial lineup, had been considered as shoo-in bets.

But suddenly, Sotto and Aquino y left early this week the Estrada-Binay camp because it reportedly selected other bets that they do not approve of.

Oreta said she felt betrayed in that she stuck with Estrada during his Senate impeachment trial, but Estrada instead reportedly chose her nephew, Tarlac Rep. Benigno "Nonoy" Aquino III, on the opposition’s slate.

Nonoy and his mother, former President Cory Aquino, actively sought Estrada’s resignation on allegations of graft. This led to the aborted impeachment of Estrada at the Senate in January, 2001, and Malacanang’s takeover by then Vice President Gloria Arroyo.