The chartered Boeing 727 had just leveled off after takeoff from the Madison, Wis., airport when the normally staid and inaccessible Walter F. Mondale appeared at the entranceway to the back section of the plane.

Without a word, a nearly expressionless Mondale began to shower the press corps with mushy, purple grapes. When his straw basket was empty, he turned and went back to his seat in the front cabin.

Occasionally, Mondale is given to such shows of what passes for glee. He had just left the largest rally of his presidential campaign, and the fifth in a row with big, enthusiastic crowds.

"I think we're moving. There's something bubbling in American life now," he said in an interview moments after throwing grapes. "Now I believe we're in this race, and we've got an excellent shot at it."

Until last Sunday's debate with President Reagan, Mondale was, by his own account, frustrated and discouraged. "Negative feelings about me" -- the sense that he was "wooden and joyless and grim and dull" -- muzzled any issue-oriented message he offered, he said.

But in the debate, many Americans warmed toward him, he said, and revised their views about "Reagan the president -- his leadership, his vision, his grasp of the facts, his concepts."

The ebullience is not all emotion. After weeks of occasionally bumbled events, Mondale advance teams have come up to snuff.

Each rally site is saturated with Mondale-Ferraro signs and carefully placed rows, and even fan-spreads, of American flags. The few Reagan-Bush or anti-abortion demonstrators who show up are usually placed well out of sight, if not out of sound.

Mondale has two not entirely new speech-writers -- Harry S Truman and Will Rogers -- whose words are spicing his rhetoric. Audiences are catching on to his invitation to get involved by answering rhetorical questions or joining in on catchy refrains.

When Mondale leaves here Monday to campaign in St. Louis and then California, only 22 days will remain before the Nov. 6 election.

As Mondale moves toward his second and final debate with Reagan next Sunday in Kansas City, his strategists are trying to shift the campaign focus to the subject of that debate, foreign policy, generally considered Reagan's weaker suit.

Vice President Bush gave them what one strategist described as a virtual godsend in that regard, when during his debate with Geraldine A. Ferraro he accused the Democrats of saying the Marines killed by terrorist attacks in Lebanon had "died in shame." Neither Mondale nor Ferraro has made such a statement.

Mondale labeled Bush's statement "unpardonable" and has demanded an apology from Bush. But the vice president has declined to do so, and for the last two days Mondale has been pressing the issue in increasingly stronger terms.

"I was and am angry as hell," Mondale said in the interview. "If people are going to believe that, they're going to believe that a candidate for president of the United States from one of the two major parties would be such a low-life, so utterly contemptuous of human respect and honor for these kids that he would do something like that.

"The shame was that these kids weren't protected . . . . They didn't die in shame; they died with honor . . . .What makes me angry is that Americans have certain things that unify us . . . .

"Would you vote to put anybody in public office who had said those kids died in shame in Beirut? I wouldn't. If I had said something like that, I would withdraw."

Mondale has steadfastly refused to join the chorus of those who question whether Reagan, at 73, might be too old to be president. "I have no way of evaluating that . . . ," Mondale said.

But it is "valid," he said, to raise questions about Reagan's ability to handle the job.