It was the perfect symbol for the final days of an imperfect but undaunted campaign:

As Geraldine A. Ferraro waved goodbye to well-wishers in Illinois earlier this week and strolled across the airport tarmac, she suddenly spotted most of her senior advisers standing behind the police honor guard, waving back and wearing mustachioed Groucho Marx masks.

Although all portents point to Ferraro & Co. winding up at the bottom of a landslide in next Tuesday's general election, her campaign continues its high-stepping strut toward the finish line -- a chipper "death march," as one aide put it.

Except for occasional snatches of gallows humor, it is impossible to tell from the behavior of Ferraro and her staff that the Democrats are trailing by 12 to 24 percentage points in major polls. If anything, the giddy behavior and relentless stumping suggest more the whiff of victory than the stench of defeat.

Rather than ease up on the schedule, Ferraro is intensifying a brutal final week that includes whirlwind visits to 11 states in four days. Sixteen-hour days are being stretched to 18, as the candidate tells onlookers again and again, "I believe we're in a fight for the heart and soul of the country, and I truly believe we're going to win."

One reason for the high spirits is a belief within the Ferraro camp that the candidate has fought the good fight, that regardless of the election returns, she belongs to the ages as a historic figure who has effectively served her running mate, her party and her gender.

"I am," she told talk-show host Phil Donahue in Chicago Tuesday, "a credible candidate."

Particularly because Ferraro continues to draw large, boisterous crowds, there is great reluctance to give up the ghost.

"This isn't like brain surgery. Public opinion is very volatile. . . . To say that any election is over before it's over is absurd," issues director Steven Engelberg said. "People are coming up to us and asking why we did so poorly among women. What are they talking about? The election's Tuesday!"

There are other reasons for ebullience. Ferraro is running in the second slot on a ticket that has been an underdog from the start and, if the Democrats lose, the defeat is not likely to be seen as a direct referendum on her candidacy.

Furthermore, virtually everyone on Ferraro's staff has a life and job away from national politics, which does not diminish the desire to win but seems to make the potential defeat less devastating.

Win, lose or draw, the Ferraro campaign appears destined to end in a blaze of humor and high jinks.

At the end of one particularly arduous day this week, campaign press secretary Francis O'Brien and Engelberg spotted a news magazine correspondent snoozing slack-jawed on the airplane. Snickering mischievously, they maneuvered a wide-awake competitor into the adjacent seat and had her type diligently while pictures were snapped comparing the two reporters.

Ferraro generally has kept a sharp edge in her stump speeches. Today in Wisconsin, she paused in a driving rain long enough to deliver an acerbic challenge to debate "one-on-one" with President Reagan, who said her selection for the Democratic ticket was based more on sex than qualifications.

Nevertheless, little gaffes appear to be mounting amid the campaign grind.

Tuesday night, during a rally at Drexel Hill Middle School outside Philadelphia, Ferraro inadvertently urged the crowd to vote for Ronald Reagan. Catching her error, she grimaced, grinned and said sheepishly, "So you know I'm human, right?"

Today, after working a crowd at Cappuccio's Meat and Sonny's Cut-Up Poultry in Philadelphia's Italian market, Ferraro introduced Rep. Barbara B. Kennelly (D-Conn.), "one of my closest friends."

Although Kennelly's name sounds Italian, she is Irish, Ferraro said, before proceeding to misspell it, saying K-I-N-N-E-L-L-Y.