Moon, a former head of the main opposition Democratic Party, is currently running neck and neck with UN Secretary-General Ban Ki-moon in public polls among potential candidates as the next president. Yoo is also included in the list, although his approval rating hovers in the single digits.

Yoo said Moon's protests against the deployment of an advanced U.S. missile defense system in the country lead to doubts about his views on national security.

While South Korea claims the battery will improve the country's defense capabilities against the North, the opposition bloc has been claiming it will only lead to unwanted diplomatic tension with Beijing.

Moon also has been under fire for allegedly playing a part in contacting Pyongyang and getting feedback that was then reflected in Seoul's decision-making process on a UN vote, Yoo added.

A memoir by a former Foreign Minister Song Min-soon released earlier this year claimed Seoul abstained from voting on the 2007 UN resolution on North Korea's human rights after hearing Pyongyang's views, and that Moon was involved in contacting the North.

Yoo also cited Moon's remark where he said he plans to visit North Korea before flying to Washington if he becomes president.

Concerning speculation he does not always see things in the same way as Rep. Kim Moo-sung, who is a major shareholder in the new party, Yoo stressed that while there are minor differences, he strongly trusts Kim.

The lawmaker then said he hopes Rep. Na Kyung-won of Saenuri will join the new party. Na earlier claimed she would leave Saenuri but later said she needed more time to determine the sincerity of the new party.

"Na said she will join the party in January, but I understand there are some different media reports," Yoo pointed out.

The first group of 29 lawmakers, along with another who left Saenuri earlier, vowed to establish a new party Tuesday, expressing discontent with the loyalists of President Park Geun-hye, who was impeached earlier this month over corruption.  (Yonhap)