Senate Foreign Relations Committee members in a 5 1/2-hour closed session yesterday, grilled Ernest W. Lefever, President Reagan's nominee as assistant secretary of state for human rights, on alleged conflicts of interest.

Committee members predicted that they would vote to send the nomination to the floor today with a recommendation that it be disapproved.

However, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) predicted that lefever can be confirmed by the full Senate "after a difficult struggle."

In a news conference, Baker said that he expected a Democratic filibuster and that Democratic support would likely be needed not only for a two-thirds vote necessary to close debate but also for the majority needed to confirm Lefever.

The nominee was questioned closely by committee members as to possible conflict of interest in his relations with the Nestle Corp., a large manufacturer of infant formula. The Ethics and Public Policy Center, headed by Lefever, accepted $25,000 from Nestle and $10,000 from its attorney while distributing information supporting Nestle's marketing of infant formula in Third World countries.

Nestle had been attacked by development experts and health officials for aggressive practices that induced poverty-striken Third World mothers to abandon breast feeding for formula, which is often excessively diluted or mixed with polluted water.

Millions of babies have suffered from malnutrition as a result, according to social activists who have organized a worldwide boycott against the company.Nestle officials, however, say their product has alleviated hunger and helped many women unable to breast feed, especially in drought areas.

While Republicans are generally backing Lefever under White House pressure, many are lukewarm in their support, as indicated by Foreign Relations Committee Chairman Charles H. Percy's decision to appoint Sen. S.I. Hayakawa (R.-Calif.) to manage the nomination on the Senate floor. Hayakawa, known for his tendency to fall asleep in meetings, is one of the least experienced and influential members in floor leadership.

Sen. Richard G. Lugar (R-Ind.) said he would vote for Lefever because he is Reagan's candidate, but added he would have preferred "a stronger advocate" for human rights. Lefever, he said, "is not a warm and outgoing political type person. . . . The ideal nominee would be a person who had a very warm outflowing feeling for humanity, someone like Pope [John] Paul. But Pope [John] Paul is not available."

Besides Lefever's relationship with Nestle, a central issue is his belief that the U.S. government should press for human rights more ardently in unfriendly Communist countries than in nations such as Guatemala and Argentina, which are strategic allies.

Several committee members also questioned Lefever on statements by two of his brothers that he believes blacks are genetically inferior. Lefever had denied he ever said or believed that, however. His ties with South Africa have been under scrutiny, but are not a major factor in the opposition to his nomination.

Even Lefever's critics said he kept his cool under hostile questioning yesterday. He was "more responsive and better prepared," said Sen. Alan Cranston (D-Calif.). Sen. Joseph R. Biden Jr. (D.-Del.) said Lefeve "is totally incompetent on every score, but he's hanging tough."

Cranston said the Democratics have not taken steps to organize a filibuster on the Lefever nomination. However, he added, "Our doubts were strengthened, not weakened, by the testimony."

Sen. Christopher J. Dodd (D-Conn.) said Lefever's testimony on his Nestle connection "clearly indicated a strong appearance of conflict of interest."

The floor vote will be "very, very close," said Cranston who has taken his own nose count. But he acknowledged that the choice of Hayakawa would make his job easier.