A bruising presidential campaign was drawing to a close in the Philippines on Friday, with a last-minute attempt by the president to unify candidates against a front-running mayor perceived as a threat to democracy virtually collapsing.

After crisscrossing the archipelago nation for three months, five presidential candidates led by Mayor Rodrigo Duterte will converge in the vote-rich capital Manila for their final rallies ahead of Monday's vote.

On the eve of the final day of campaigning, President Benigno Aquino III made a desperate call on candidates to agree to an alliance to defeat the brash Duterte, who has been likened to US Republican front-runner Donald Trump for his controversial remarks but has topped pre-poll surveys.

Duterte's 30-point lead in surveys can be overcome if his trailing rivals mainly former Interior Secretary Mar Roxas and Sen Grace Poe join hands, Aquino said, implying that some of them should back out and support a single aspirant. Under the Philippine electoral system, a candidate who gets the most votes is proclaimed the winner, even if no one gets a majority.

Poe, however, refused an invitation by Roxas, who has been backed by Aquino, to meet and discuss an arrangement where she would be forced to back out. Vice President Jejomar Binay also stated he would not step aside. Sen Miriam Defensor-Santiago, who has trailed far behind in surveys, has also vowed never to surrender.

Duterte's camp said calls for an alliance against him "reeks of stench of defeat." "It's an admission that a victory by Mayor Rodrigo Duterte has virtually become inevitable," said the mayor's national campaign manager Leoncio Evasco Jr.

A longtime mayor of southern Davao city, the 71-year-old Duterte courted controversy with his profanity-laden speeches, vulgar jokes and devil-may-care irreverence but has successfully tapped into public insecurities with a bold promise to wipe out crime and corruption in three to six months if he is elected.

Aquino, business executives and church leaders felt that he crossed the line when he joked about wanting to have raped first an Australian missionary, who was gang-raped and brutally killed by inmates in a 1989 jail riot. When the Australian and U.S. ambassadors sniped at his joke, Duterte asked them to shut up and expressed openness at the possibility of severing ties with major Western allies if he wins the presidency. He also has threatened to close Congress if lawmakers try to impeach him if he wins next week and has said he would allow Marxist guerrillas to play a political role in his government.