This July 4 file photo shows Rep. Lee Jae-myung of the main opposition Democratic Party. (Yonhap)

Rep. Lee Jae-myung, former presidential candidate of the main opposition Democratic Party (DP), will declare his bid for the party's chairmanship on Sunday, an aide said.

The DP plans to hold a national convention on Aug. 28 to elect a new leader, and whether Lee will run for the party leadership has been a point of contention within the DP amid a power struggle between pro- and anti-Lee lawmakers.

Supporters say Lee is the right person to lead the party, while opponents argue that he should take responsibility for the party's defeat not only in March's presidential election but also in June's nationwide local elections.

While announcing his run for the contest, Lee plans to emphasize the importance of political reform and party unity, the aide said. (Yonhap)