Campaigning for vacant House of Councillors seats started Thursday in Nagano and Hiroshima prefectures, with the outcomes likely to affect Prime Minister Yoshihide Suga's leadership and his plan to call a general election that must be held by fall.

A by-election for the Upper House district in Nagano Prefecture and a do-over election for the constituency in Hiroshima Prefecture — both scheduled for April 25 — will be the first national polls since Suga took office in September last year.

The two contests will effectively be vied between candidates of the ruling coalition of Suga's Liberal Democratic Party and its ally Komeito, and those backed by opposition parties.