Opinion polls on Tuesday show incumbent leader Megawati Sukarnoputri lagging behind her former chief Security Minister, General Susilo Bambang Yudhoyono.
Susilo is the clear frontrunner to take over the world's most populous Muslim nation on a platform of firm leadership, cleaner government and faster economic growth.
More than 147 million people will be eligible to vote on 5 July across Indonesia, a sprawling archipelago of 17,000 islands and 220 million people.
A legislative body previously chose Indonesia's presidents.
Indonesian Democratic party
Running mate: Hasyim Muzadi, chief of country's largest Muslim organisation.
Running mate: Jusuf Kalla, former minister and business tycoon.
Running mate: Salahuddin Wahid, deputy of Indonesia's human rights commission and senior Nahdlatul Ulama leader.
National Mandate party
Running mate: Siswono Yudhohusodo, chief of
the Indonesian Farmers Association.
United Development party
Running Mate: Agum Gumelar, transport minister
The three main presidential candidates are secular nationalists. Two others are Muslim leaders.
But polls show Yudhoyono winning 40% of the vote, easily beating Megawati or his other main rival, former military chief Wiranto, the nominee of Golkar, once the party of former president Suharto.
If no candidate wins a majority, a runoff will be held in September between the two top candidates.
Megawati has stabilised Indonesia after chaos that followed Suharto's ousting in 1998, but many see her as too weak to eradicate profiteering and improve the rule of law.
Her party came second in parliamentary elections on 5 April after losing nearly half its seats. Golkar was first, but won less than a quarter of the seats on offer.
Yudhoyono's fledgling party won 10% of the seats. Since that time, he has shot to presidential favourite.