The Senate, sitting as an impeachment court, on Wednesday ruled that only senator-judges able to take part in the trial should be counted when determining the threshold for conviction.
Four of the 24 senator-judges have been unable to take part in the proceedings and cannot vote.
The decision means prosecutors now need 14 votes for a conviction, instead of the 16 votes required when all 24 are taking part.
Some senators had argued to keep the 16-vote threshold regardless of the four absences, which would have made it more challenging to convict Duterte.
The impeachment trial follows a high-profile fallout between Duterte and her former ally and running mate President Ferdinand Marcos Jr and the outcome could shape the 2028 presidential election, which Duterte intends to contest.
Marcos is limited by the constitution to a single term in office and is expected to back a successor.
Duterte, a frontrunner in early opinion polls, faces a ban from holding political office if convicted by the court and would be out of the presidential running.
She stands accused of misusing public funds, amassing unexplained wealth and threatening the lives of Marcos, the first lady and a former House Speaker.
Duterte denies wrongdoing.
"This is the best chance to secure a conviction," Michael Henry Yusingco, a senior research fellow at the Ateneo Policy Center in Manila, said of the court's decision to lower the voting threshold.
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