The ruling Liberal Democratic Party's closely watched leadership race is set to be announced Monday, kicking off the 12-day official election campaign period ahead of the Oct. 4 voting by party lawmakers.

It will be the first LDP presidential election since the party and its coalition partner, Komeito, lost their combined majority in both chambers of the country's parliament.

The parties lost their control of the House of Representatives, the all-important lower chamber, in last October's general election, and of the House of Councilors, the upper chamber, in the election in July this year.