The Democratic Party of Japan and Ishin no To (Japan Innovation Party) will hold a convention March 27 in a Tokyo hotel to launch their new party, members of the two parties said Monday.

The largest and third-largest opposition parties will merge to challenge Prime Minister Shinzo Abe's ruling coalition in the Upper House election this summer.

"We have to upgrade the DPJ to stop the runaway government of Abe and realize a change of power," DPJ leader Katsuya Okada told representatives of his party's local chapters Monday evening.

Okada and Ishin no To leader Yorihisa Matsuno signed a merger accord last Friday, with the DPJ chief saying he will serve as head of the new party until the Upper House election.

Altogether, about 150 lawmakers from both chambers of the Diet will join the new party, but it will still be far smaller than the ruling coalition of Abe's Liberal Democratic Party and Komeito, which controls a majority of the 242-member Upper House and more than two-thirds of the 475-member Lower House.