Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) has warned the White House that the nomination of Dr. Ernest W. Lefever to be assistant secretary of state for human rights is in serious trouble, Senate sources said yesterday.

Baker issued that warning Thursday, the sources said, even before there were new signs of trouble for the nomination. Yesterday, Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) authorized the committee staff to reopen inquiries into the financing of the public policy center that Lefever heads, while two other Republian members of the committee announced their opposition to the nomination.

The defection of the two Republicans -- Larry Pressler of South Dakota and Rudy Boschwitz of Minnesota -- appeared to doom Lefever's chances in the committee. However, this would not necessarily preclude an effort by Lefever's most ardent conservative backers to force the issue to the Senate floor, where they believe they may have a chance of prevailing.

Despite these new developments, administration officials led by President Reagan rallied to Lefever's side, while the embattled nominee told reporters on Capitol Hill that he saw "no problem" in winning Senate confirmation.

"I'm committed," the president said of the Lefever nomination as he left the White House to spend the Memorial Day weekend at his ranch in California.

The State Department, meanwhile, issued a statement saying that Secretary of State Alexander M. Haig Jr. "has reiterated his complete support of Dr. Lefever's nomination."

The statement also branded as "erroneous" a report in The Washington Post yesterday that said the department was reviewing the nomination. At question is whether Lefever's influence in determining the U.S. policy on marketing infant formula in the Third World was a conflict of interest because of his public policy center's financial relationship with formula manufacturers.

Yesterday's statement by the State department said there has been and is no review of the Lefever nomination. Department spokesman Dean Fischer said the only thing that could be called a review was a written explanation of his role in the infant formula matter that Lefever prepared at the request of the Foreign Relations Committee.

However, at a late afternoon news conference, Haig, while again expressing "confidence in Dr. Lefever," conceded that the department "is very much aware" of the conflict of interest allegations that have been made against Lefever and is "looking into them."

The United States this week was the only country to vote against a voluntary international code to restrict the marketing of infant formula and other breast milk substitutes. During his confirmation hearings, Lefever acknowledged discussing this issue with U.N. Ambassador Jeane Kirkpatrick, a member of the Cabinet-level group that recommended the policy.

Later, after checking his recollection with Kirkpatrick, Lefever said they made only a "passing reference" to the policy at most and that any such discussion took place before his nomination and before he began working on an unsalaried basis pending confirmation at the State Department.

While publicly the administration was reiterating its support for Lefever, some officials at the White House yesterday expressed private concern over the possibility that a close reexamination of Lefever's financial dealings could led to a series of embarrassing revelations.

Lefever's "casual attitude" in dealing with corporate and foreign contributions to his Ethics and Public Policy Center is a source of some concern to these officials, one source said.

But just the sort of reexamination that the White House would prefer to avoid may have been set in motion yesterday by Foreign Relations Chairman Percy, who was privately asked the White House to consider withdrawing the nomination. Senate Republican sources said Percy has agreed to a request by Democratic committee members to recall Lefever for questioning in private June 1.

The committee will decide after the June 1 session whether to reopen public hearings on the conflict of interest questions, the sources said. Committee staff aides have already begun interviewing potential witnesses.

Three Democrats on the committee -- Paul E. Tsongas of Massachusetts, Alan Cranston of California and Christopher J. Dodd of Connecticut -- yesterday called for Lefever to withdraw his name from consideration. They said Lefever's "misleading and evasive testimony should disqualify further consideration for a post which requires a forthright advocate and champion of human rights."

New opposition also surfaced yesterday from Republican committee members and from the American Jewish Committee. Pressler said approving Lefever "would cast the Republican majority in the Senate in a mold that I don't think we should be cast in."

"As a witness, he has certainly demonstrated that he is the wrong man to handle this most delicate aspect [of foreign policy]," said Boschwitz.

Maynard Wishner, president of the American Jewish Committee, added in a letter to Haig that Lefever was now so damaged that the best interests of the country would be served if the nomination were withdrawn.

A spokesman said the State Department considered the opposition of the committee to be "very significant."