(From L-R) Former foreign minister Seiji Maehara, former transport minister Sumio Mabuchi, Trade Minister Banri Kaieda, Finance Minister Yoshihiko Noda, and Agriculture Minister Michihiko Kano join hands at the start of a political debate for the ruling Democratic Party of Japan's (DPJ) leadership race in Tokyo August 28, 2011. Japan's next leader, to be selected in a DPJ election on Monday, faces huge challenges including a resurgent yen that threatens exports, forging a new energy policy while ending the worst nuclear crisis since Chernobyl, and finding funds to rebuild from a devastating March tsunami as well as to pay for the ballooning social welfare costs of a fast-ageing society. [Photo/Agencies]

TOKYO - Japan's ruling Democratic Party started Monday an election of the party's president, who is expected to become the country's third prime minister in two years.

Five candidates, economy, trade and industry minister Banri Kaieda, former foreign minister Seiji Maehara, finance minister Yoshihiko Noda, agriculture, forestry and fisheries minister Michihiko Kano, and transport minister Sumio Mabuchi, are vying for the top post to succeed outgoing Prime Minister Naoto Kan.

Kan resigned as party chief Friday after seeing the passage in the Diet of two key bills - one on the issuance of debt-covering bonds and one on the promotion of renewable energy. Kan is the longest-serving premier in five years.

A total of 398 DPJ lawmakers are eligible to cast ballots after the final presentations by the candidates starting from 11 a.m..

Banri Kaieda is now seen as the front runner, thanks to the backing of party powerbroker Ichiro Ozawa, who has the largest intraparty faction.

But local media and political observers say no single candidate is likely to garner the 200-vote majority needed to win, and a run-off is highly possible, in which case Kaieda is expected to face off one of the anti-Ozawa candidates, namely fiscal hawk Noda or security hawk Maehara, who share the similar power base in the party.

Whoever wins the election will have to cope with challenges including reviving a stagnant economy, rebuilding the disaster - devastated areas, ending the world's worst nuclear crisis since Chernobyl in Fukushima, arresting a surging yen which threatens export, and seeking cooperation from oppositions parties in a divided diet.