Sara Duterte impeachment: Why the Senate is fighting over the 16-vote threshold

Manila: A key legal and procedural question in Vice President Sara Duterte’s impeachment trial has shifted the Philippine Senate’s attention from the evidence to a basic question: How many senators must vote to convict?

The 1987 Constitution clearly states an impeached official cannot be convicted without the concurrence of two-thirds of "all members" of the Senate.

With 24 senators, the Senate impeachment court initially interpreted on July 6, 2026 that provision as requiring 16 votes for conviction.

But four senators have not been participating in VP Duterte’s trial.

On Sept. 8, Senator Erwin Tulfo sought a reconsideration of the 16-vote threshold.

The Senate impeachment court then voted to reconsider whether the constitutional phrase “two-thirds of all the members of the Senate” should always mean the full 24-member chamber, or whether senators who are legally or physically unable to participate should be excluded from the calculation.

At the opening of the trial on July 6, Senate impeachment court presiding officer Francis “Chiz” Escudero ruled that 16 votes would be required to convict Duterte — two-thirds of the Senate's full 24-member membership. No senator-judge initially sought reconsideration.

That means:

The arithmetic is straightforward; the constitutional question is who belongs in the denominator.

Four members of the 24-seat Senate have been unable to participate in the trial:

The Senate has therefore had 20 active senator-judges for the proceedings.

That creates the central legal dilemma: should the four senators who cannot hear the evidence and participate in deliberations nevertheless remain part of the mathematical base for determining the two-thirds requirement?

House prosecutor Chel Diokno argued on Wednesday (Sept. 23) that the full 24-member Senate should not automatically be used as the denominator.

His argument: an impeachment judge must be able to take an oath, hear evidence, deliberate and vote.

If a senator is legally incapable of performing those functions, Diokno argued, that senator should not necessarily be counted in calculating the threshold.

The prosecution described a fixed 16-vote requirement as potentially producing what it called an “acquittal by default” if senators who cannot participate are nevertheless included in the denominator.

Importantly, Diokno also argued that changing the denominator does not automatically favour the prosecution.

Using the numbers cited by the House, if all 24 senators count, 16 votes are needed to convict.

If only 20 qualified participants count, 14 would be needed.

The prosecution's position is therefore that the constitutional requirement remains two-thirds; what is disputed is what “all the members” means in the circumstances of this trial.

Duterte's defence has urged the impeachment court to retain the 16-vote threshold.

Lawyer Michael Poa argued Wednesday that Escudero already ruled on July 6 that 16 votes were required and that no senator-judge appealed that ruling at the time.

The defence says there has been no “supervening event” that would justify changing the rule more than two months into the trial.

The defence has also raised due-process concerns, arguing that changing the threshold during the trial could alter the rules governing Duterte's possible conviction.

The Senate invited four retired Supreme Court magistrates to serve as amici curiae, or friends of the court, to provide legal views.

Three — former Chief Justices Hilario Davide Jr., Artemio Panganiban and Reynato Puno — said the two-thirds requirement should be calculated using senators legally and physically able to participate in the trial.

Retired Associate Justice Adolfo Azcuna took the opposing view, saying the Constitution refers to all members of the Senate and that a senator remains a member unless the seat becomes vacant through events such as death, resignation or expulsion.

Panganiban also argued that senators who did not participate in hearing the evidence should not be counted as judges because they would not be in a position to make an informed judgment.

Article XI, Section 3 of the 1987 Constitution gives the Senate the sole power to try and decide impeachment cases.

The dispute is whether that authority allows the Senate impeachment court to interpret the phrase “two-thirds of all the members of the Senate” for the unusual circumstances of the current trial.

Some senators have argued that the Supreme Court should resolve the constitutional question instead.

Senators Alan Peter and Pia Cayetano, for example, said they would not participate in the threshold vote because they believe the issue should be left to the Supreme Court.

On Wednesday, the impeachment court first rejected an attempt by Sen. Alan Peter Cayetano to prevent it from taking up the threshold question.

The vote was 13-6, with one abstention among the 20 senator-judges present.

That vote did not determine the number needed to convict Duterte; it only allowed the threshold issue to proceed.

The court then proceeded to oral arguments on whether the 16-vote requirement should be reconsidered.

The significance is substantial:

If the Senate retains the full 24-member denominator, 16 votes remain necessary to convict.

If it decides that only participating or legally qualified senator-judges should count, the number needed for conviction would fall.

The precise alternative threshold depends on which senators the court ultimately decides should be included.

This is not simply an arithmetic dispute.

The constitutional text deliberately sets a two-thirds "supermajority" for conviction in an impeachment trial.

The unresolved issue is whether that fraction should be applied to the Senate's fixed membership of 24 or to the number of senators legally capable of participating as judges in this particular proceeding.

That distinction could materially affect the number of votes required at the eventual judgment.

The threshold vote itself is not a vote on whether Duterte should be convicted.

It answers a separate legal question: what number constitutes the constitutionally required two-thirds in the circumstances of this trial?

The eventual conviction vote will come later, after the impeachment proceedings and presentation of the parties' cases are completed.

As of September 23, 2026, the original rule was 16 of 24 senators. The Senate impeachment court is now reconsidering whether absent or legally unable-to-participate senators should be included in the denominator. That decision could change the number of votes required for conviction, but it does not itself determine Duterte's fate.