President Carter's political strategists firmly believe he will win reelection because the American electorate will conclude that Republican nominee Ronald Reagan is not of presidential caliber.

They are convinced that they can make Reagan's personal characteristics the central issue of the campaign and in so doing exploit what their polling data show is the Republican's most serious problem -- public doubts about him on such "presidential" qualities as judgment and steadiness.

But as the conclusions of the Democratic National Convention here Thursday night showed, the president begins the race against Reagan on the defensive, leading a troubled party that shows few signs of enthusiasm for his style of leadership.

Carter's strategists have no illusions about the lack of enthusiasm for the president or the extent of public dissatifaction with his performance in office. Their planning assumes that both Carter and Reagan are burdened by high negative ratings, but that in the end the president's problems will prove less debilitating than his opponent's.

To exploit what his strategists consider Reagan's serious weaknesses when viewed as a potential president, Carter will hammer away throughout the fall on the central theme that if he is not to sit in the Oval Office for the next four years, it is Reagan -- and only Reagan -- who will.

It is not exactly a strategy of high inspiration.

"The question in the fall is who is the bigger turkey," said one irreverent administration official. "I think people will decide that Reagan is."

The president's acceptance speech at the convention here was a road map to the themes he will use this fall.

Carter, whose job ratings are at an all-time low, said in his speech that the election of 1980 is less a referendum on his record than it is a "stark choice" about the country's future.

"The president is a servant of today," he said. "But his true constituency is the future. That is why the election of 1980 is so important."

Carter's theme, that he and Reagan represent "two futures," reflects a view of political reality in 1980 that is shared by his strategists and others. v

A Democratic pollster outside the Carter campaign said before the speech that such an approach represents the president's best chance of surviving the fall. If Carter is forced to defend the America of 1980 -- with its inflation and unemployment and foreign policy troubles -- he will lose, the pollster said.

But if he can focus attention on the kind of president Reagan would be and how the country might change under his leadership, Carter is much better off, he said.

Reagan is vulnerable on this score, the Carter advisers say, because polling data show that the people have the most doubts about him on those qualities they take most seriously in choosing a president -- his judgment and ability to deal with complex issues, for example.

"The life of every human being on earth can depend on the experience and judgment and vigilance of the person in the Oval Office," the president said in his acceptance speech. "The president's power for building and his power for destruction are awesome. And the power's greatest exactly where the stakes are highest -- in matters of war and peace."

The president will couple this with an effort to give the electorate, in the words of one Carter strategist, "a door to walk through" once enough doubts are raised about the GOP nominee.

Voters need more than reasons to vote against Reagan, he said. They need a reason to vote for the president, and the Carter campaign will try to provide it by concentrating on a limited number of Carter successes in an attempt to persuade voters that Carter has not been as ineffective a president as so many of them seem to think.

This message will be carried to the nation in a paid media campaign that will take two approaches, one positive and the other intrinsically negative.

The positive side will stress the Carter record and suggest to voters that, through no fault of their own, they may not be aware of some of the president's more important accomplishments.

The negative side, which is likely to become more overt as the campaign heats up, will stress personal qualities in the president chosen for their assumed contrast with Reagan's most serious vulnerabilities.

Carter will be portrayed as a vigorous, hard working chief executive -- the age issue. And Carter will be seen grapling with complex problems, meeting alone with foreign leaders -- the judgment and intelligence issue.

As the president left New York today for a few days of rest at Camp David, Md., he was in a position somewhat similar to that of President Ford at this time four years ago. Then Ford was 30 points behind the Democratic nominee and facing a seemingly impossible task. Yet on election day, he almost pulled it off.

But Carter campaign aides say their internal polling data shows that Reagan does not enjoy anything like the advantage Carter held over Ford in the summer of 1976. They expect the president to rise in the post-convention polls and for the gap between him and Reagan to narrow to about 10 or 12 points in September.

Then the contest will be viewed as a real race and the pressure -- the kind of pressure, they stress, that Reagan has never experienced in all his years in politics -- will begin to build on the GOP front-runner.

Critical to the success of the "start choice" approach the Carter forces are taking to the campaign is the elimination of independent candidate John B. Anderson as an alternative. So long as Anderson is around, even voters who become convinced that Reagan is unacceptable as president do not necessarily have to opt for Carter.

Tim Kraft, the president's campaign manager, predicted that by mid-September Anderson's standing in the polls will sink to 5 or 6 percent. But even at that level, Anderson would pose a serious problem for Carter in several key states, Kraft conceded. All the Carter campaign can do to counter this, he said, is to emphasize that "a vote for Anderson is a vote for Reagan."

The Carter campaign is also betting that the president will gain more than Reagan from the fall debates between the candidates. The nationally televised debates are an ideal format to reinforce the idea that the election comes down to a choice between two men, which is one reason Carter at first balked at any joint appearances with Anderson.

The president will not begin his campaign until Labor Day. He will not, his aides say, try to duplicate the Ford "Rose Garden strategy" of 1976. But while he will campaign more than Ford did, Carter will also make full use of-the White House as a backdrop to display the "presidential" qualities his aides say Reagan lacks.