Trustees of the League of Women Voters will meet today to decide whether independent John B. Anderson will be invited to participate in the league-sponsored debate scheduled for Sept. 21 in Baltimore.

President Carter has sought to arrange a head-to-head encounter with GOP candidate Ronald Reagan as the first debate, but Reagan has refused any invitation before the league-organized event. Reagan advocates including Anderson, who draws more votes from Carter than from the former California governor, according to polls.

The three leading candidates agree that the first debate of the campaign is the most important, because it is likely to attract the largest TV audience. For Anderson, a good debate showing might receive his campaign; excluson from the debates might make it impossible for him to improve on his current standing.

Anderson predicted yesterday that the league will invite him and that Carter will agree to participate, despite some hints by Carter aides that the president might boycott the debate if he cannot meet Reagan alone.

Anderson was cheered by a new poll published in Time magazine showing him with the support of 15 percent of voters -- exactly the threshhold the league set for candidates to win inclusion in the debates. Carter and Reagan ran even with 39 percent each in the poll. The Time poll is the third recent nationwide poll that has put Anderson at or above 15 percent.

At what a league spokesman said is likely to be an all-day meeting today, the league's executive committtee will study recent poll results and get technical advice on interpreting polls.

The three leading presidential candidates will be advised of the league decision privately, and the decision will be made public Wednesday, the spokesman said.

Carter aides have complained privately that the league favors admiting Anderson and that it will juggle its complicated criteria to ensure his participation.

Press secretary Jody Powell and campaign chairman Robert S. Strauss said separately yesterday that they have no assurance from Reagan of a head-to-head debate even if they agree to an initial three-way debate.

"If we agree to a three-way debate there's a growing feeling here that there will never be a one-on-one debate," Powell said.

"Carter can win [the election] without debating. I hope we don't have to do it," Strauss said.

Carter's representatives have pointed out that he is the first incumbent president to agree to debate not only his major rival, but a third candidate as well. They have accused Reagan of hiding behind Anderson and trying to minimize the number of debates by letting campaign time slip by while refusing to discuss any debate except the first league debate. The league also plans two subsequent debates.

James Baker, one of Reagan's senior advisers, has said that Reagan prefers to have only two presidential debates. He has told reporters that the second of these can exclude Anderson and follow the form of the Lincoln-Douglas debates, in which there were no questioners and only the candidates spoke.

Anderson, campaigning in upstate New York, told a news conference that Carter has an obligation to meet him in debate, and the president's refusal would create a potent new issue over "the right of the American people to hear another voice."

Anderson said his attorney sent the league a letter two weeks ago presenting his case for inclusion in the debates, but has made no further effort to lobby the league.