The Philippine leader has called on presidential candidates to agree to an alliance to defeat a front-running mayor, who he describes as a threat to democracy and has built a reputation for his profanity-laden speeches.

President Benigno Aquino III said Friday the 30 percent lead of Mayor Rodrigo Duterte in pre-election surveys could be overcome if his rivals, mainly former Interior Secretary Mar Roxas and Sen. Grace Poe, can unite.

Aquino did not elaborate but he was insinuating some candidates could back out and support a single candidate to surmount Duterte's lead.

The brash Duterte, who has been compared to Donald Trump in the U.S., has caused alarm with his threats, including of closing Congress if lawmakers try to impeach him if he triumphs in the May elections.