It was the first time the PDI-P of ex-president Megawati Sukarnoputri has led in any poll since campaigning started last month, with most giving the Democrats a clear advantage heading into the April 9 election.

The survey by the Strategic Centre for Development and Policy Review also showed that none of the main parties - the PDI-P, the Democrats and Golkar - would achieve the 20 per cent of the vote required to nominate a presidential candidate on its own.

The PDI-P had 19.60 per cent support ahead of 19.18 percent for the Democrats and 18.26 per cent for Golkar, the largest party in Mr Yudhoyono's parliamentary coalition.

The findings mean the parties will have to join forces with smaller parties to field a candidate in the presidential elections in July, institute executive director Husin Yazid said.

'There will be stiff competition among the three nationalist parties and they will have to form coalitions with smaller parties in order to nominate their own candidates for president,' he said.

The survey, conducted among a sample of 1,250 voters, also found that 35 to 40 per cent would not vote, or would lodge informal or incorrect ballots. About 22.5 per cent were undecided. The poll had a three per cent margin of error.

'Many voters are confused as there are too many parties and they don't know the legislative candidates well,' Mr Yazid said.

Daniel Sparringa, a political analyst at Surabaya's Airlangga University, said the survey's results could give a second wind to Islamic parties, which are seeing their support dwindle. 'Islamic parties are still important. The nationalist parties need to join forces with them... to build harmony and gain symbolic power,' he said.

The vote is the third general election in Indonesia, the most populous Muslim-majority country in the world, since the fall of military strongman Suharto in 1998. -- AFP