Prince Norodom Ranariddh is the leader of the royalist Funcinpec Party and son of Cambodia's King Norodom Sihanouk.
Prince Ranariddh was forced to flee Cambodia after the 1997 coup
His campaign has centred on his
royal connection - a major attraction to older voters.
But he has also promised significant public sector pay rises, and has repeatedly criticised the increasing number of Vietnamese nationals living illegally in Cambodia.
Unlike his main challenger Hun Sen, who has largely remained silent during the campaign period, Prince Ranariddh has been vitriolic in his attacks on the other main parties.
He told the BBC that the elections could not be free and fair because his rivals were intimidating and coercing voters.
Funcinpec has been plagued by defections since its poor showing in last year's local elections, when it narrowly avoided losing second place position to the Sam Rainsy Party (SRP).
Prince Ranariddh, who was born in Phnom Penh in 1944, is the eldest son of King Sihanouk and a minor wife.
1944: Born in Phnom Penh, as King Sihanouk's eldest son
1983: Entered politics as his father's Bangkok representative
1991: Became head of Funcinpec
1993: Won general election, but forced to share control
1997: Ousted in coup
1998: Lost to CPP in general election
He studied and practiced law before his father encouraged him into politics, and he took the king's place as the head of Funcinpec in 1991.
In 1993, Funcinpec won the general election, but Hun Sen refused to cede power and a ruling partnership was agreed between the two leaders.
Relations remained strained, and both sides angered the other by trying to win support from smaller parties.
Prince Ranariddh's deal with Khmer Rouge leaders who had defected from the government was the final straw for the CPP, analysts say.
In July 1997, Hun Sen was behind a violent coup which ousted the prince and forced him to leave the country.
In his absence, Prince Ranariddh was found guilty of arms smuggling, but he was pardoned by the king and returned to Cambodia shortly afterwards.
In the 1998 elections, Funcinpec was heavily defeated by the CPP, but allegations of electoral fraud triggered violent protests.
A deal was eventually agreed on, with Funcinpec becoming the junior party in a coalition government, and Prince Ranariddh himself becoming the head of the National Assembly.