TOKYO (Reuters) - Yokohama voters were casting ballots for mayor on Sunday in an election expected to affect unpopular Prime Minister Yoshihide Suga's prospects for keeping his post amid a COVID-19 surge across Japan.

       A loss by the Suga-backed Liberal Democratic Party candidate in Suga's home turf just south of Tokyo would pile pressure on the premier, whose approval ratings slid below 30% in August, fanning concerns in the LDP about his ability to lead into a general election this year.

       Suga, who took office last September after predecessor Shinzo Abe quit citing ill health, is struggling to contain the pandemic, with national daily infections hitting a record 25,000 last week.

       Hachiro Okonogi, backed by Suga, is running in a crowded field of eight including the incumbent mayor, two former governors and opposition Constitutional Democratic Party of Japan candidate Takeharu Yamanaka, a professor of public health.

       Although Yokohama is in Suga's constituency in parliament, a survey conducted by a local newspaper in mid-August found Okonogi trailing Yamanaka, reflecting concerns over the government's handling of the pandemic.

       Kanagawa, the prefecture where Yokohama is located, has been under a state of emergency since Aug. 2, but infections have continued to rise. The prefecture recorded a record 2,878 cases on Friday, according to public broadcaster NHK.

       Suga's term as LDP president ends in September. A general election must be held by Nov. 28.

       (Reporting by Sakura Murakami; Editing by Linda Sieg and William Mallard)