President Carter and Ronald Reagan, by finally agreeing to a one-on-one debate, have transformed the 1980 campaign from an intricate chess match into a single roll of the dice with the White House at stake.

The decision has scrambled schedules and strategies, as each side now begins a new round of psychological warfare against the other. Carter, who has wanted a face-to-face encounter with Reagan all fall, began it yesterday when he tried to downplay the debate's significance.

Asked by the Associated Press if he thought the debate would be decisive in the campaign, he said, "No, I think not, except to the extent that it will define the issues more clearly which have not yet been defined adequately in the campaign so far."

The president also tried to lower expectations of those who expect him to defeat Reagan in the defeat. "I don't know about winning," he said. "I'm a careful enough observer to know that Gov. Reagan is a professional in dealing with the media. He's articulate and I don't underestimate him."

This psychological struggle is likely to show up again tomorrow, when the two sides open negotiations on a time, place -- and perhaps sponsor -- for the debate.

The League of Women Voters suggested Oct. 28 in Cleveland when it issued invitations to Reagan and Carter Friday. Reagan's advisers seem willing to accept that date -- though not necessarily league sponsorship -- but Carter's advisers prefer an earlier meeting, perhaps Oct. 25 or 26, in part because they feel the momentum is now with the president and they do not want to lose it.

As one Reagan adviser said yesterday, the agreement to meet "will freeze the campaign where it is until the debate, with Reagan slightly ahead."

Reagan appears likely to go into a stall on the stump until the debate. "No one is likely to risk anything with a such a major event coming up," said a Reagan aide. "We don't think Carter will risk anything."

But Carter's advisers don't want to let Reagan take advantage of the interregnum. "We are going to work hard not to allow that freezing effect to take place," said one Carter campaign official yesterday.

It is possible that Carter, who is scheduled to be campaigning in the Northeast and the South tomorrow, Tuesday and Wednesday, will add a day or two of campaigning in an attempt to prevent the presidential race from going into suspended animation between now and the debate.

Both campaign staffs see the debate useful for strategic reasons, despite the obvious risks to both candidates, and each side believes it will benefit from a debate that is judged a draw.

For the president, the debate means an opportunity to draw distinctions with an opponent who has hugged the center stripe through most of the fall campaign. "Coming out of the convention, a number of voters believed there were no major differences between the candidates or they did not understand the degree of those differences," said one Carter campaign official. "These were a number of voters saying that it wouldn't make much difference who was elected. Whether we win in a debating points sense or not, this is the single best opportunity to solve that political problem."

The concern that Carter's use of the incumbency would enable him to make dramatic use of a foreign policy issue to dominate the last days of the campaign was a decisive argument among Reagan strategists for the former California governor's agreement to the debate.

"It's a useful insurance policy in case something big on foreign policy comes out of the White House," one Reagan aide said.

That offset the chief argument against meeting Carter head-to-head, which was that it was risky for Reagan to debate when he had the lead. Many of Reagan's field coordinators have long advocated a debate, however, believing Reagan would show well against the president, and several yesterday reacted positively to the candidate's decision.

"I don't like just sitting there when the lead is that close," said Don Totten, Reagan's Illinois field coordinator. He said polls showed Reagan with a 3 percentage point lead in Illinois and a persistent problem with some women voters on the war-peace issue.

Moreover, there are conflicting reports on where the undecided vote is going. Reagan's chief of staff Ed Meese and pollster Richard Wirthlin insist the undecided vote is moving toward Reagan, while Carter strategists have said it is going their way.

Carter strategists, who had pressed for a one-on-one debate with Reagan, now say they do not expect the president to overwhelm his Republican opponent, but they do expect him to demonstrate his mastery over "the complexities of issues" and believe that will work to his advantage in a face-to-face encounter. They also expect Carter to avoid a fatal blunder.

Reagan's advisers also expect him to hold his own against Carter, but are considering ways to overcome any blounder. "We will put a lot of things in a holding pattern until we see how he does," said one Reagan aide yesterday. "If he did badly in the debate, we might have to come back with a heavy negative [advertising] campaign. But we are not expecting that."

Both campaign staffs are now drawing up new contingency plans, depending on how the debate schedule works out. If the debate is held Oct. 28, both candidates will have to redraw campaign schedules. Carter had planned a last-week blitz to begin Oct. 27, while Reagan had scheduled appearances in New Jersey, Pennsylvania, Illinois and Texas beginning next weekend.

Reagan's advisers, fearing Carter might use the preparation period to make news from the Rose Garden, are considering having Reagan hold a press conference to make news while the two men are off the campaign trail preparing for the debate. "We don't have a Rose Garden," said one Reagan adviser, "but we ought to be able to do something."

Vice President Mondale and Reagan's running mat, George Bush, may play a more important role in the closing days, especially if Carter and Reagan go into hibernation to prepare for the debate.

Meantime, Reagan spent a perfect fall football Saturday campaigning through Lincolnland -- traditionally Republican central Illinois -- in an effort to improve his chances of carrying the 26 electoral votes of that swing state.

His itinerary was carefully designed to touch all bases, from a college homecoming parade, to a visit with farmers and Teamsters at a highway truckstop, to a courthouse rally, to a visit to Lincoln's tomb, to a trip down a coal mine, to a stop at a family farm -- and, finally, a rally beneath the arch in St. Louis.

Friendly crowds greeted him at every stop on the cool, sunny day, with the only discordant note coming from supporters of the Equal Rights Amendment.