Come May, more than 50 million Filipinos will make a choice that will have an impact on their lives for the next six years.

The man who has given them a period of prosperity not seen in decades - Mr Benigno Aquino - will step down after serving as president for six years. The Constitution limits his rule to a single term.

All five candidates seeking to replace him are promising the same things: to combat corruption, create jobs, raise salaries, bring down prices, open more doors to investors, cut taxes, fix Manila's horrendous traffic problem, and blanket the entire archipelago with free Wi-Fi. But it may not be the promises that will prove pivotal, but the election itself.

"I think this will be among the most unpredictable elections in decades. In the past eight months, we have had three seemingly runaway leading candidates, only to see the shifts and shocks in their fortunes," De La Salle University political analyst Richard Javad Heydarian told The Sunday Times.

A looming scenario is that the country will have a president who lacks either legitimacy or a mandate, leading to a bumpy period that may deal a blow to the economy.

Senator Grace Poe, the leading contender, is dogged by legal challenges to her candidacy.

The election commission disqualified her over allegations that she has not been a resident in the Philippines long enough to qualify her to run for president. A Supreme Court injunction has allowed her to stay in the race, but three justices have already opined that she is not qualified to run. Her name may not even be on the ballot come election time.

Philippine Vice-President Jejomar Binay, who came out on top in the latest survey along with Ms Poe, may be in jail even before the first ballot is cast, as he faces a string of corruption charges.

Mr Rodrigo Duterte, a former mayor who has electrified voters with pledges to lead a brutal war on crime - "I say let's kill five criminals every week, so they will be eliminated" - missed the deadline to file his candidacy. At least one case has been filed to bar his bid to replace a candidate whom his party fielded earlier.

Senator Miriam Santiago, a stage 4 cancer survivor, has had to fend off speculation that she may not last six years to serve out her term - that is, if she wins, which is a long shot.

That leaves former interior minister Mar Roxas, the standard-bearer of Mr Aquino's Liberal Party. But his polling numbers are barely moving, so his chances of winning rest largely on Ms Poe and Mr Binay being out of the race. If he does win, he will carry the burden of an unwieldy mandate.

Pundits are already predicting a return to the turbulent years when the now-imprisoned Gloria Arroyo was president. She faced allegations that she won in 2004 through massive cheating. During her term, she had to deal with two military revolts and four impeachment cases, all of which hampered her efforts to push growth above 4 per cent.

But for those who see the situation as half full, it will take a colossal failure of governance to slow the momentum that Mr Aquino has set in motion. They argue that with consumption-driven growth, the economy is already too robust to be unsettled by political muckraking.

Mr Richard Bolt, Philippines director of the Manila-based Asian Development Bank, said a more crucial concern than shaky politics is the El Nino dry spell. Weather forecasters say the Philippines is in for one of the worst El Ninos on record, possibly even more severe than the 1998 drought, which wiped nearly US$5 billion (S$7 billion) from farm production.

Diplomatically, the election offers the Philippines an opportunity to reset strained ties with China.

While all five presidential candidates say they will pursue the case Manila filed before an international court contesting Beijing's claims to the South China Sea, they are also pushing for a more productive, less hostile engagement with China.

"How the next administration leverages a favourable outcome will say a lot about its foreign policy direction. If the new president decides to shove aside the arbitration outcome, then it signals... a bet on engagement with China," said Mr Heydarian.