By Francessca S. Abalos

Senator Mark A. Villar's move from the minority bloc to the Senate majority could erode public confidence in the impartiality of Vice-President Sara Duterte-Carpio's impeachment trial, political analysts said, with the shift coming amid government investigations touching on his family's business interests.

"I don't see Senator Mark's transfer to the majority as a plus," Edmund S. Tayao, president and chief executive officer of Political Economic Elemental Researchers and Strategists, said by telephone. In his view, the move could cast doubt on the Senate's commitment to weighing the evidence objectively.

Mr. Villar's office declined to comment on his decision to leave the minority bloc.

Senate Majority Leader Juan Miguel F. Zubiri denied on Oct. 9 that political calculations had driven the transfer, including any effort to lock in the 14 votes needed to convict Ms. Duterte. Senators, he said, would vote on the basis of the evidence presented during the trial, adding that Mr. Villar simply wanted to be more "productive."

Mr. Tayao said the realignment was unlikely to alter the voting threshold, which likewise sets how many votes are needed to block a conviction. He suggested the decision might instead be driven by political considerations ahead of Mr. Villar's reelection bid in 2028.

Rogelio Alicor L. Panao, a political science professor at the University of the Philippines, said impeachment votes do not necessarily decide senators' electoral prospects. He pointed to the 2001 impeachment trial of former President Joseph E. Estrada, noting that four of the 11 senators who voted against opening the second envelope containing evidence related to his secret bank account were subsequently reelected.

Joining the majority, Mr. Panao noted, can give senators greater influence over legislative priorities and access to committee leadership posts. Mr. Villar has since taken over as chairman of the Senate Public Works Committee and secured a seat on the Commission on Appointments, replacing other majority members. The commission, a constitutional body composed of senators and House members, vets and confirms key presidential appointments, including heads of executive departments and ambassadors.

The appointments come amid government investigations involving the Villar family's business interests.

"The timing of the committee appointment gives a feeling that is somewhat disturbing," Mr. Panao said in an e-mailed reply to questions. "I am afraid it would not spell good to public trust and may even reinforce the observation that the Senate operates as a transactional political arena, not an impartial court."

The Office of the Ombudsman opened an investigation on June 15 into Mr. Villar and his sister, Senator Camille A. Villar, over allegations that they delayed the Light Rail Transit Line 1 Cavite extension project. The Ombudsman has also been investigating PrimeWater Infrastructure Corp., a water utility linked to the Villar family, over alleged graft and corruption. The Ombudsman is the government's independent constitutional office charged with investigating and prosecuting alleged wrongdoing by public officials.

Giving Mr. Villar control of the Public Works committee while his family faces infrastructure-related investigations could raise conflict-of-interest concerns, Mr Panao said. The panel handles legislation on public works and infrastructure and conducts oversight of related government programs. He warned that legislative oversight could be weakened if committee leadership were used to shield private business interests from scrutiny.

Mr. Tayao said the appointment could make it less likely that the public works committee would investigate matters involving the Villar family, although other senators could still initiate hearings.

Committee appointments and political realignments coinciding with an impeachment trial, Mr. Panao said, could reinforce public perceptions that political alliances influence Senate decisions. He warned that such perceptions could weaken confidence in the chamber's ability to hold public officials accountable.

"Let us not forget that, at the end of the day, they are politicians first," he said. "We should expect them to make decisions based on political math and alliances, rather than expecting them to act as purely impartial judges."

The Senate, sitting as an impeachment court, ruled last month that only senator-judges who could participate when the verdict is rendered would be counted in determining the two-thirds majority required for conviction. The ruling replaced an earlier interpretation requiring 16 votes based on the Senate's full 24-member composition. Conviction would require 14 votes if 20 senator-judges remain eligible to participate. Under the Constitution, the Senate holds the sole power to try and decide impeachment cases, and a judgment of conviction extends no further than removal from office and disqualification from holding any office under the Republic of the Philippines.