The impeachment trial of Philippine Vice President Sara Duterte opens on Monday and is poised to define the 2028 presidential contest while widening the rift between two prominent political families that has thrown the Senate into disarray.

The result will decide if Duterte can seek the presidency and will gauge trust in the country's institutions and leaders. Experts note the crucial factor is whether citizens view the ruling as legitimate or shaped by political motives.

"If the process appears driven by partisan goals or short on credibility doubts could persist no matter the verdict" said Ederson Tapia a public administration professor at the University of Makati.

The length of the trial is uncertain with procedural arguments witness numbers and evidence presentation likely to set the pace. The 2012 impeachment of a former chief justice took four months. Conviction needs at least 16 votes from the 24 member Senate.

Surveys show Duterte leading the field for 2028 but a guilty finding could block her path to the top office.

Public trust demands fair presentation from both sides says analyst

"Should the public regard the proceedings as equitable an acquittal might bolster her standing" Tapia added.

The first such trial against a vice president involves claims that Duterte diverted public money accumulated unexplained assets and issued threats against President Ferdinand Marcos Jr the first lady and a former House Speaker.

Duterte daughter of former President Rodrigo Duterte rejects all accusations and maintains the case stems from political rivalry.

"Such claims are anticipated" said Congressman Robert Ace Barbers prosecution spokesperson. "We will allow the evidence to stand on its own."

Jean Encinas Franco a political science professor at the University of the Philippines stressed that both sides need adequate opportunity to present arguments to build public confidence.

She referenced the 2001 proceedings against former President Joseph Estrada where a Senate decision to suppress evidence sparked outrage halted the trial and triggered protests that removed him from office.

"They must ensure the prosecution and defense receive sufficient time and opportunity so the public does not see the trial as biased toward one party" she noted.

Duterte's lawyer Michael Poa stated the defense stands ready to show the claims are unfounded. He could not confirm if Duterte would appear at Monday's hearing since the summons allows representation by counsel.

From alliance to sharp division

Marcos and Duterte descendants of two major political dynasties campaigned together in 2022. Their partnership has fractured amid a growing feud that intensified after Rodrigo Duterte's arrest and handover to International Criminal Court custody in March last year.

The conflict has reached the Senate prompting questions over how the trial will proceed.

In May as the Senate prepared to accept the impeachment complaint from the House Duterte ally Senator Ronald Bato dela Rosa reappeared after months out of sight and cast the deciding vote to make Alan Peter Cayetano Senate president.

Dela Rosa then remained in the Senate before departing early on May 14 shortly after disorder and gunfire broke out in the building. His location remains unknown.

Cayetano served as running mate to Duterte's father in the 2016 election. Rodrigo Duterte awaits trial in The Hague on murder charges tied to his drug war. Dela Rosa who faces similar ICC charges led the national police during much of the campaign that left thousands dead. Both have rejected the accusations.

About a month later senators from an opposing group gathered enough backing to elect Senator Sherwin Gatchalian as Senate president.

While the internal disputes revealed shifting alliances analysts cautioned against using them as a sure guide for trial votes.

"The impeachment process sets up distinct incentives and constitutional duties compared with leadership votes or coalition deals" Tapia said.

The trial outcome may also shape the Marcos camp's choice of candidate for 2028 since Marcos cannot run again.