SEOUL, SEPT. 12 -- Roh Tae Woo, the presidential candidate of South Korea's ruling Democratic Justice Party, could bolster his political standing at home if his visit to Washington, beginning Sunday, is successful, a senior party official said today.

Hyun Hong Choo, the party's deputy secretary general, echoed statements by Roh that the hastily prepared trip is intended to reassure the Reagan administration and congressional leaders that Roh is committed to free elections in South Korea next December.

Hyun indicated that the trip also could enhance Roh's image as a democratic-minded politician, deflecting attention from his military background. But Hyun argued against the idea -- which would be politically unpopular among many voters here -- that Roh is seeking even an implicit endorsement from the U.S. government.

"We have reached the conclusion that this {visit} can really be a plus," Hyun said at a special briefing here for American reporters.

Roh renewed his pledge of fair elections today, saying he would try to convince officials in Washington that "Korea is going through a critical transition to democracy peacefully."

Roh, a former general who participated in the 1980 military coup that brought President Chun Doo Hwan to power, dismissed fears of a new military takeover. "It is true that some people are worried over the possibility of military intervention in the democratization process, and some are exaggerating it for malicious purposes," he said. "But as a man who knows the military very well, I can say with confidence that the military wishes to see my democratic reform recommendations of June 29 faithfully implemented on the basis of stability."

On June 29, Roh yielded to the pressure of nationwide protests, agreeing to a package of far-reaching reforms, including the rewriting of the constitution, the holding of direct presidential elections, the release of political prisoners and a loosening of controls on the press.

During the trip, Roh is to meet President Reagan, have talks with key congressional leaders, deliver two major speeches and hold several press conferences and interviews. His visit will be covered exhaustively by South Korea's tightly controlled media, which are expected to depict Roh as a statesman bearing few outward traces of his military career.

Despite this, western diplomats and political analysts in Seoul remain puzzled over the motives for Roh's trip, which will include Japan and a meeting with Prime Minister Yasuhiro Nakasone. The tour's timing, amid preparations for the elections, is a sensitive matter here.

Korean nationalism and anti-Americanism have been on the rise recently, but tacit U.S. support is still viewed as desirable in many political quarters.

Hyun described the request to meet with President Reagan as a political afterthought, saying Roh decided to seek the White House session after making up his mind to visit Washington at the invitation of private groups.

Hyun blamed the press for making a political issue of the Reagan meeting. The White House wavered before agreeing to the session, fearing that it could be interpreted as an endorsement of Roh's candidacy.

The Reagan administration has stressed in recent days that it is not taking sides in the Korean election. Hyun denied that a rejection of the request would have been an embarrassing rebuff to Roh, describing such speculation as "crazy."

Hyun rejected a suggestion that Roh sought the image-boosting meeting mainly to increase his power over restless factions of the ruling Democratic Justice Party and the military.

"I do not agree with the claims that {Roh} does not have a full grip on the party," Hyun said. "He certainly has a full grip. This particular visit will clear away any doubts on this point." He added that the visit will "make {Roh's} status different from other politicians'."