Japan's ruling party veteran Yasuo Fukuda and former foreign minister Taro Aso officially declared their candidacies on Saturday in the race to become the nation's next prime minister.
Their candidacy applications were submitted to the headquarters of the governing Liberal Democratic Party (LDP) in Tokyo, party officials said.
No other lawmakers followed suit and the formal deadline for applications passed a little before midday, confirming the election as a two-way battle between Fukuda and Aso.
The LDP is scheduled to select its next leader on September 23. He then becomes Japan's premier as the party controls the powerful lower house.
The 71-year-old Fukuda, a party elder seen as a foreign policy dove, has emerged as the favourite to take the top job.
"I would like to officially declare my candidacy," a confident Fukuda told reporters, having earlier said it was time to engage in "broad dialogue" with the people.
Prime Minister Shinzo Abe announced his resignation on Wednesday following a raft of scandals, a plunge in popularity and ill-health. He has been hospitalised.
The party is suffering one of the worst crises in its five-decade history after it lost control of the upper house of parliament for the first time in July.
The newly invigorated opposition is doing its best to block key legislation -- including logistical support for US-led operations in Afghanistan, on which Abe had effectively staked his job.