Support for South Korea in Congress has declined so drastically that President Carter's proposal to leave weapons behind as insurance when U.S. troops pull out could not now pass the House, a congressional foreign affairs expert said yesterday.

Rep. Clement J. Zablocki (D-Wis.), chairman of the House International Relations Committee, said it would be futile even to begin hearings on the President's plan this year because of fallout from the Korean influence buying scandal.

In connection with his plan to withdraw American ground forces from Korea beginning next year, Carter last week asked Congress to approve a transfer of $800 million worth of American weapons to the Korean Army.

Zablocki said approval of the proposal is probably impossible until the Seoul government starts cooperating with American investigations of the Korean effort to win friends and influence in the U.S. government.

Zablocki's committee moved yesterday to increase the pressure on South Korea by unanimously approving a resolution that blames the influence-buying scheme on the government of President Park Chung Hee and demands Seoul's cooperation in the investigations.

The resolution was quickly scheduled for floor action Monday. The full House-seems certain to pass it.

The Korean embassy here has said that any efforts to win influence in Washington were thw work of private individuals with no ties to the Seoul regime.

Last week, after three days of hearings on the question, the House Committee on Standards of Official Conduct reached a different conclusion. The committee's counsel said the bribery effort was an "official plan" of the Korean government.

Whatever the Korean government may have done in the past, it seems clear that the scandal and Seoul's failure to cooperate have seriously eroded its support in Congress.

Since the end of the Korean War the Asian nation has enjoyed warm backing from healthy majorities in both houses of Congress. Not even reports about repression of human rights seriously impaired Korea's status as a congressional favorite.

A month ago, however, the House came within 15 votes of cutting off U.S. aid to Korea beacuse of non-cooperation with the influence-buying probes.

Zablocki's warning yesterday about Carter's weapon-transfer plan suggests that Korea's standing has declined further since that vote.

The transfer plan seemed to have broad support when Carter first disclosed it months ago. Members of Congress who oppose the U.S. troop withdrawal agreed that, if troops were to leave, military aid to Korean should increase. Those who support the troop withdrawal saw the weapons transfer as a wise insurance policy against attacks on South Korea.

Yesterday, though, Zablocki told reporters that "the climate in the House, the attitude of the members, is such, because of the Korean matter, that favorable action would be impossible."

Interviews with House members from both parties indicate that the climate in the House is more one of fear than of fury.

Some members expressed outrage that an established ally would use corrupt means to influence votes in Congress. A larger number expressed only the worry that any apparent support of South Korea now could have harmful political consequences.

Members are skittish these days about anything that says 'Korea' on it," said a relatively junior Democrat. "They probably wouldn't oppose aid to Korea - they'd just rather not vote for it right now."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said yesterday that he had not planned to bring the weapons-transfer proposal to the House floor this year in any case. He said the White House "wanted us to hold some hearings on it and then take it up next year."

Zablocki said hearings would not be held this year. He was unclear about when he would take up the measure. "We have to have some progress on the Korean investigation" before the legislation can pass, he said.

Jerrold Schecher, press aide to Carter's national security adviser, Zbigniew Brzezinski, said the proposed troop withdrawed from Korea would begin next year whether or not Congress had approved the weapons transfer plan.

Carter's plan, as set forth in his message to Congress, calls for a phased withdrawal of ground troops over a four to five-year period, with phased transfer of American arms to Korea as the troops leave.