Bob Kur, a correspondent for NBC News, is a regular passenger on the DC9 that is whisking Vice President Mondale back-and-forth across the country this campaign season.

Sitting in the midsection of the plane, Kur whistles the theme from "The Twilight Zone" and then, in a Rod Serling imitation, asks, "Whatever happened to Bob Kur? Has he crossed the border beyond time, where . . ."

Kur, like many of the 20 other news people aboard Mondale's plane, was seen regularly in the nightly news shows during the abortive presidential campaign of Sen. Edward M. Kennedy.

But since being assigned to cover President Carter's running mate after the Democratic convention, Kur's mellifluous words have been heard on the NBC Nightly News only once, on Aug. 28.

He has spent the last month "in the box," which is trade jargon for "off the air."

It's not that the vice president isn't working hard. He is. In three days last week, he made speeches, in order, in New York, Wisconsin, Minnesota, Rhode Island, Massachusetts, Maine and Connecticut.

And he got the desired result, from a political point of view. The local newspapers and radio and television stations duly recorded his words.An aide to the vice president said that the strategy is "all business is local. There are 51 little elections, and we're out here doing politics."

Everywhere Mondale went, he lavished praise on the location and its local Democratic candidates.

Sometimes, his good work had preceded him. When the vice president spoke in the community room of Portland's Franklin Towers for senior citizens (at 16 stories the tallest building in Maine), several people thanked him for the $1.8 million modernization grant that had been announced a few days earlier for another Portland housing project.

While hyperbole and pork barrel may please the folks in Bridgeport, Portland, Providence, Minneapolis, Boston, La Crosse, New Haven, and two Rochesters (New York and Minnesota), all of which the vice president visited within 60 hours, they can't compete with pronouncements by President Carter, or the president's rivals for the Oval Office.

"Let's face it," says Susan Spencer, one of the two CBS correspondents traveling with the vice president, "when Carter is campaigning, why should we go with number two, when we can get it straight from the top banana?" s

Spencer, a rising star at CBS, became a familiar face to millions of Americans while covering the Kennedy campaign, making almost daily appearances on the air. Since joining the Mondale roadshow, she hasn't been on the Cronkite show in more than a month.

Lem Tucker, the other CBS correspondent trailing Mondale, has had "a couple of close calls," but has appeared on the Cronkite show only once this month. (The double coverage is ordered so that if Mondale does make news, one reporter can stay behind and file a report while the other keeps up with the candidate.)

Most of the network correspondents have a contract that provides staight salary, regardless of how often they appear on the air. But ABC's George Strait gets paid for each on-air appearance.

"Since Labor Day, that has been zero" for the ABC Evening News, said Strait, grimacing. But he's managed to augment the rent money by filing stories for ABC radio and 12 feeds to "Good Morning America."

"But George would trade six of those morning spots for one shot at night," said Tucker. (Strait recently got on the nightly show when the Titan missile exploded in Arkansas, because Mondale was speaking that day in nearby Hot Springs).

Andrea Mitchell of NBC, like Kur a former Washington television reporter, watched John Chancellor the other night on a TV set in the Warwick home of former Rhode Island governor Philip Noel. "Three energy stories -- my old beat -- after none for six months." groused Mitchell.

Later, however, she admitted "a preference to be on the campaign. I love it." And even if she isn't on the air often, "we are fulfilling a role" by protecting the network.

"Sure you want to be on," Mitchell said, "but only if there is something good. You have to keep on reporting. You can't start slumping."

It was in that spirit that Susan Spencer stayed behind in Minneapolis when the rest of the Mondale entourage flew on to New England Tuesday. She thought Mondale's speech to the state AFL-CIO convention in Rochester, Minn., might warrant 30 or 60 seconds that night.

She and her crew, which includes a producer and camera and sound operators, took a commercial airliner to Chicago, where they "fed" a film story to New York, and then poised themselves in front of a TV set, hoping to see the fruits of their labors on the Cronkite program. They didn't.

Undaunted, Spencer and her crew grabbed an 8 p.m. flight to Boston, where they picked up the trail, still hoping for a break -- a blunder, gaffe or, most unlikely, honest-to-goodness news -- that would get them "out of the box" and on the air.

The fate of ABC's Strait was even worse that day. While the other members of the press corps found time for a good dinner and a night's sleep at a comfortable hotel, Strait and his crew kept an all-night vigil outside the Boston hospital room of the ailing Rose Kennedy.

As the plane -- by now dubbed Morpheus 2 because of its sleepy news mission -- approached yet another airport last week, an aide to the vice president confided that his boss will make "a major speech of education" that surely will rate national news coverage.

He was wrong. The network correspondents are still in the box.