By Heekyong Yang and Hyonhee Shin
SEOUL (Reuters) - South Korea's ruling party is expected to win a sweeping victory in Wednesday's regional elections, exit polls suggest, an extra boost for President Moon Jae-in following the summit between North Korean leader Kim Jong Un and U.S. President Donald Trump.
The ruling Democratic Party was on path to win most key constituencies, including Seoul mayor, in a vote in which some 4,000 seats were up for grabs.
A landslide win would be a coup for Moon, who took office in May last year on a platform of reconciliation with the North and enjoys a support rate of over 70 percent.
Kim vowed to work for denuclearisation, and Trump promised security guarantees for the North at the Singapore summit.
Moon played a mediator role between Kim and Trump, having already held two summits with Kim in April and May.
Support in pre-election polls for the ruling Democratic Party far outpaced the conservative opposition, which has been under intense pressure to reform after a political scandal that ousted Moon's predecessor, Park Geun-hye.
Park is currently serving a 24-year prison sentence after having been found guilty of charges including bribery, abuse of power and coercion.
According to the exit polls, compiled by three major South Korean broadcasters, the Democratic Party was seen likely to secure 14 out of the 17 major municipality chiefs, including Seoul mayor, who is often considered to be the next presidential candidate, and 10 parliamentary seats.
Complete results are due on Thursday morning.
(Additional reporting by Christine Kim; Editing by Nick Macfie)