For two presidential candidates who have long professed themselves eager to give the American voter the opportunity to see them in a debate, President Carter and Ronald Reagan did a lot of backing and filling last week.

The debate over the debates is an issue Reagan welcomed for two reasons.

It replaced Reagan's own embarrassing debate with his running mate, George Bush, over China policy as a campaign issue, and more importantly, it is an issue on which he has the leverage.

Time is on Reagan's side and so is the "fairness" principle that the Republican candidate found so popular when he tried to open up a debate between himself and Bush to other GOP candidates at Nashua, N.H., during the primary campaign.

Reagan wants independent John Anderson in the first debate. Carter does not.

Even though Reagan campaign adviser James Baker conceded to reporters last week that the Republican's stand is not based on high moral principal, Baker said he is certain the American people will react adversely toward anyone who tried to exclude Anderson.

Time works for Reagan because the GOP candidate wants only two debates. By insisting (for reasons that don't stand up to scrutiny) that it would be inappropriate to discuss, let alone accept, any forum other than the first League of Women Voters debate in Baltimore about Sept. 21. Reagan lets the days pass by, shortening the calendar in which debates might be held.

Carter has tried to counter this tactic by accepting three non-League invitations, but as fast as the president accepts debates, Reagan swats the invitations away.

The president has looked awkward discussing debates ever since the Anderson independent candidacy brought his initial response that he would not debate Anderson under any circumstances.

When that proved Politically unpopular, Carter agreed that Anderson could join some debates but insisted that the firt debate be a head-to-head confrontation with Reagan.

Both sides know from past compaigns that the first debate has the widest audience and influences more people's voting decisions than those that follow.

The Carter camp cannot wait to put their candidate on the stage with Reagan so that he can throw up to his challenger some of the quotes from Reagan's political past. In their speeches at the Democratic National Convention, Vice President Mondale and Carter gave a preview of the collection of Reagan quotes their research staffs have accumulated.

Anderson's presence in the first debate does two things for Reagan. It protects him from a direct confrontation with Carter, and it makes possible a pro-Anderson surge if the Illinois GOP congressman does well.

Robert Strauss, Carter's campaign manager, said last week that according to polling, Anderson takes seven votes from Carter for each vote he takes away from Reagan.

In May, Carter accepted the League's debate invitation without qualification. Now, he appears to be boxed in, with no way to force an encounter with Reagan prior to the League's first debate.

Strauss hinted last week that Carter might skip the League appearance if he doesn't get his way, leaving Reagan and Anderson to debate, but that course seems politically suicidal.

Carter can hope that Anderson, whose campaign is stumbling, falls below 15 percent in the polls and thereby fails to qualify for the League debate.

But Carter's aides believe that the League would prefer Anderson in than out and will jiggle the complicated criteria it has set up to enable Anderson to participate.

Ruth Hinderfield, head of the League's Education Fund, indicated that the League's board of directors was free to alter the criteria and that it might be willing to have Anderson in one debate but not a second.

That would give Reagan the three-man debate he wants first and then the head-to-head Reagan-Carter debate that the leading candidates say they want, but that only one, Carter, tries to do anything to arrange.

If the first debate is held as Reagan wants it, the leverage could switch to Carter.

It would be the president, then, who would be pressing for more rather than fewer debates, and it would be the president who had not yet had the type of debate that he wanted.

More attention might then be paid to Reagan's reason for wanting only two debates -- his need to do other types of campaigning. That need is less convincing when Reagan's campaign schedule is examined. Five of the first 16 days of September are scheduled as days off.