President Reagan and his top aides yesterday made a major new commitment to winning Senate confirmation of Ernest W. Lefevr, and brought the controversial State Department nominee to the White House for a high-level embrace and strategy session.

"I haven't retreated one inch from wanting him," Reagan told reporters about his embattled nominee to be assistance secretary of state for human rights.

White House chief of staff James A. Baker III underlined the administration's all-out support for Lefever by inviting him to a meeting at the White House yesterday to plan for the next round of Senate Foreign Relations Committee hearings on the nomination.

At the meeting, White House deputy counsel Richard Hauser, who was asked several days ago to review conflict-of-interest charges against Lefever, reported to Baker and Lefever that he had found no conflict, according to White House staff director David Gergen.

The White House stepped up its effort for Lefever's confirmation on the same day that Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) told the president that confirmation would be "a tough fight."

At a White House meeting, the senator said he told Reagan that a likely Democratic filibuster against the nomination would force pro-Lefever forces to come up with 60 votes, the three-fifths of the Senate required to cut off a filibuster.

Sen. Alan Cranston (D-Calif.), a leading opponent of Lefever, said, "We may have the votes to beat him without a filibuster. We're considering everything."

Lefever is by far the most controversial of Reagan's nominees because of his greater tolerence for human rights abuses in countries friendly to the United States than in hostile nations and his connection with the Nestle Corp., a major producer of infant formula.

The White House has now thrown its support more strongly behind him than behind any other nominees whose Senate confirmations are being held up for various reasons.

"We think it's important to back our nominee. We are stepping up the pressure," Gergen said. He described the meeting in chief of staff Baker's office at the White House as "an effort to bolster" Lefever's candidacy. The White House chief of staff usually does not participate in strategy sessions with presidential nominees.

The chief of staff also did some personal lobbying for Lefever by calling Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) to discuss the next hearings on Lefever set for Thursday.

In addition, the White House instructed its chief congressional liaison, Max L. Friedersdorf, to get involved to the Lefever fight. The State Department will continue to play the leading role, but Friedersdorf will consult and coordinate with the department, a White House official said.

White House deputy press secretary Larry Speakes said the White House was aware of Lefever's relationship with Nestle before Lefever was nominated.

Lefever's Ethics and Public Policy Center received $25,000 in contributions from Nestle to distribute an article that supported the international marketing of infant formula.

Many health officials around the world argue that the marketing of baby formula in developing countries creates an unnecessary risk by convincing poor mothers not to breast-feed their babies, but to use the formula, which can be mixed in unsanitary conditions and diluted excessively to provide less nutrition.

Lefever has been working without pay at the State Department pending his confirmation, and questions have been raised about his involvement with the Reagan administration's decision to cast the U.S. vote against a World Health Organization nonbinding code of conduct opposing the advertising and marketing of infant formula in developing nations.