A nightmare is haunting the Carter campaign, even in this moment of the president's convention triumph.

In the nightmare, it is the last week of October. The final presidential debate is past. Carter did not "destroy" Ronald Reagan, as Jody Powell and others had privately predicted he would. But he did plant serious doubts about Reagan's understanding of world problems, doubts that were heightened by Reagan's twice misspeaking or confusing Indochina and Indonesia and then saying North Korea when he meant North Vietnam.

The published polls have narrowed to a four-point Reagan margin, and John Anderson's strength has been declining ever since he got into a shouting match with Lane Kirkland, the AFL-CIO president, who accused him to his face of having "a worse labor record than Reagan."

Pat Caddell's private tracking data -- based on 500 phone calls a night -- are telling Carter that his support will pass Reagan's on Saturday, Nov. 1, and -- barring a last-minute reversal -- he should be a couple of points up by Election Day, Nov. 4.

But, also, Caddell is counseling his boss that the turnout factor is still worrisome; turnout is something even the best pollsters can only estimate and, if the actual turnout is at the low end of Caddell's probable range, Carter will lose.

A student of polls and political behavior, Carter does not need to be told why. A Democrat in a close race like his, he knows, must not only persuade voters to support him; he must mobilize the persuadable and move them to the voting places. Republicans will vote of their own volition; Democrats have to be "voted."

That is why Carter is embarking on this final, frenetic sprint at the end of what has been already a grueling campaign -- to stir the crowds and build the turnout.

From Camp David, where he has been given one night's rest, he heads south to Florida, stopping in Jacksonville and Miami -- aiming to lock in his narrow lead and 17 electoral votes.

It is in Texas the next day that the trouble begins. His planners have committed two full days to the traditional itinerary, running him "up the valley" in south Texas to turn out the Chicano vote, then bringing him into Houston for a huge closing rally.

They have to have Texas and its 26 electoral votes to win. Everything else of consequence west of the Mississippi is headed toward Reagan. Early in October, Carter had made two feints into California in hope of making Reagan squander some resources to protect his home state. But now it is too late for feints. The West, including California, is giving Reagan its 103 electoral votes.

But Carter has gone no farther than Brownsville, Tex., when the frantic calls from Washington headquarters reach him. They want to change his schedule.

According to plan, he is not scheduled back into the border South in these final days, believing that Jerry Rafshoon's "Dixie pride" TV ads would hold southerners behind the southern candidate, as they did in 1976. But now reports have come in that the George Bush-Howard Baker joint tour is moving Tennessee back into the Republican column, and Jesse Helms' campaigning is threatening to do the same thing in North Carolina.

If their 23 electoral votes are lost, along with those of Virginia, Mississippi and Louisiana -- which were written off earlier -- then Carter will have only 61 assured votes from the South. And that is not enough.

But there are worse problems. After leaving Texas, Carter is scheduled to spend the last 100 hours in five states he has to sweep to have a chance of winning: Wisconsin, Michigan, Ohio, Pennsylvania and New York. Half the time will be lost to travel and sleep, so there are just 50 hours to secure 125 electoral votes.

It seems ridiculous that four years of governing, 11 weeks of campaigning should all come down to this. But that is the way it has to be for a president who began the campaign with no real political base.

The first reading after the Madison Square Garden convention in mid-August had counted only eight states with 65 electoral votes as "safe" for the president. And even some of those, like Hawaii and West Virginia, were placed in that category more on the basis of their voting history than their current polls.

By contrast, Reagan began with more than 200 of the 270 electoral votes he needed almost guaranteed. "I know how Jerry Ford felt in 1976," Carter said to someone. "We had him beat so bad coming out of convention hall, he never could catch up."

Back then, Ford and his running mate, Bob Dole, had been forced to squander valuable campaign time securing such normally Republican states as Indiana, Iowa and Kansas. This year, Carter and Vice President Mondale were confined for days to seeking support in places like Massachusetts and Maryland, where a single brief visit would normally sacrifice.

Because of that, the big and vital swing states felt neglected and, in this last, anxiety-ridden week, ultimatums came thundering down: Michigan is gone unless Carter's planned Macomb County stop is expanded to hit Flint and Saginaw as well. Both ends of Pennsylvania need work -- and a single stop in the black wards of Philadelphia will not be enough.

New York Democrats, who have been a pain to Carter from the start, are demanding two days in that state, since Texas is getting two. And similar pleas, threats and premature alibis are coming from Ohio and Wisconsin.

The real crusher, though, is the finding -- in Caddell polls -- that states that ought to have been locked up weeks ago are still shaky. "Can you believe," Ham Jordan asks, "that Ed Muskie can't guarantee Maine and Fritz isn't even sure of Minnesota? And Rhode Island -- you know we could lose this damn election in Rhode Island! No Democrat ever loses Rhode Island.

"I sure wish Election Day didn't come so early this year," Jordan says. "God, what I would give for another three days."

This nightmare is a fantasy. But the Carter people fear it could become a reality.