Republican presidential nominee Ronald Reagan last night rejected a proposal by the League of Women Voters for a two-man debate next month between him and President Carter.

The league's proposal, a change from its previous position, was made earlier in the day, and had been accepted by Carter.

The new invitation, which essentially confirmed to the position insisted on by Carter campaign throughout the negotiations about the debates, was for a package of two or more debates, the first between Carter and Reagan and a second also involving independent candidate John B. Anderson.

Arriving in Portland, Ore., last night after a day of campaigning in the West, Reagan said of his stand on further presidential debates remains unchanged.

"I cannot in good faith agree to . . . a series of presidential debates that would preclude John Anderson from debating President Carter in the same or similar way that I debated Mr. Anderson," Reagan said.

He added that he hoped the league would return to "its former principled position" of insisting that all three men be included in the debates.

Reagan's proposal is for a series of round-robin, two-way debates that would include all three candidates and be determined by a coin flip.

Reagan said that James A. Baker III, a senior campaign official, had informed the league that he could not accept the latest proposal.

"It basically restates the Carter position," Baker said earlier. "There is nothing new in it except that the league has caved in to fairly substantial pressure."

Anderson yesterday accepted the invitation to the proposed three-man debate in late October, but his campaign's general counsel, Mitchell Rogovin, said, "We are disappointed that the League of Woman Voters has chosen, in an apparent effort to appease President Carter, to change its format and provide for a two-man debate between Carter and Ronald Reagan."

The league's new package of two more presidential debates was the latest development in the maneuvering by the three candidates for an advantage on the debate issue. It represented a victory for Carter, who until now has borne the public relations onus of the man who would not debate.

The onus may have shifted to Reagan now that he has turned down the latest invitation from the league, sponsor of the 1976 presidential debates.

The president's strategists, believing that Anderson will take more votes from Carter than he will from Reagan, have sought to diminish Anderson's public standing and his potential impact in the election. To that end, they have insisted that Carter first debate Reagan alone before he would agree to a three-way debate.

The Reagan strategists, making essentially the same political calculation, have insisted that Anderson not be excluded from the debates.

The League of Women Voters at first in effect sided with Reagan and Anderson. The organization adopted a criterion that any candidate with the support of 15 percent of the electorate in public opinion polls would be invited to its debates.

As a result, all three candidates were invited to the debate last Sunday in Baltimore. But only Anderson and Reagan showed up after the president insisted on first meeting Reagan alone.

In issuing the new invitations yesterday, league President Ruth J. Hinerfeld conceded that they involved a "bending" of the league's criterion on Anderson. According to a Harris survey released today, Anderson's support has risen to 19 percent since the Baltimore debate.

But Hinerfeld, suggesting that the league saw no alternative if it was to be the sponsor of any other debates this year, said there were "pragmatic reasons" for the shift in policy and that the league was primarily interested in affording "the public a chance to see all the candidates."

Hinerfeld also said that if Anderson dropped substantially below the 15 percent support level by the time of the proposed three-way debate, "we would take another look" at allowing him to participate. This is precisely what the Carter strategists hope and predict will happen.

The league also invited the three vice presidential candidates to a debate on Oct. 2 in Louisville. Vice President Mondale and Anderson's running mate, former Wisconsin governor Patrick J. Lucey, accepted the invitation, although Mondale said he had a scheduling conflict with the date. Baker said that the response of George Bush, Reagan's running mate, is likely to be affected by Reagan's decision on the presidential debates.

The league's proposal was for Carter and Reagan to debate the week of Oct. 12 in Portland, Ore., with the three-way debate including Anderson to take place the week of Oct. 26 in Cleveland.

Baker said that Reagan may be willing to debate Carter alone later in the campaign, depending on the political circumstances.