Sen. Edward M. Kennedy (D-Mass.), in the tradition of his newly certified underdog status, tried yesterday to press President Carter into holding a "joint discussion" at a conference both men are scheduled to address separately next Thursday.

The president, in the tradition of front-runners, refused.

And a Kennedy aide, in the tradition of campaign aides, promptly left open the possibility that the Democratic challenger just might drop by the meeting of the Consumer Federation of America early anyway, perhaps at 11 a.m., which is when the president is to speak.

The day's political potboiling began with Kennedy sending the president a carefully framed letter suggesting that the annual consumer assembly would be "an excellent opportunity for us to have the joint discussion which was planned to occur in January," a reference to the Iowa debate appearance that Carter cancelled. The White House heard of the letter first from reporters, and deputy press secretary Ray Jenkins said Carter would have to decline again, because the appearance would be "political," and the crisis in Southwest Asia remained of prime concern.

The exchange in Washington came as Kennedy left town to begin the first leg of an economy-minded campaign swing to New England and elsewhere.

Its luxurious charter jet shot down by a decimated campaign treasury, the Kennedy entourage hit the road by commercial plane and bus. It was Kennedy's first long trip since his big loss in Iowa and the major speech Monday designed to give his campaign a new and more liberal foundation.

The charter jet was not the only aspect of the pre-Iowa Kennedy effort missing yesterday. The candidate also junked his old stump speech, a generalized indictment of "weak leadership" in the White House, and replaced it with specific points taken from Monday's speech.

Speaking to a labor conference in Philadelphia and a student audience at Northeast University in Boston, Kennedy repeated his demands for immediate imposition of gasoline rationing and wage-price controls. He repeated his warnings that Carter's "exaggerated reaction" to the Soviet invasion of Afghanistan will "carry us right back into the Cold War."

Although Kennedy now has to juggle his schedule to meet airline timetables, and his traveling staff has been reduced from 15 to five to save money, everything went off without a hitch on this 13-hour campaign day.

Those most disturbed by the switch to commercial flights were Kennedy's Secret Service detail.

When Kennedy's charter was in service, the agents could control every person and piece of luggage entering the plane. They used a German shepherd, which would sniff each bag for explosives.

But there was no dog yesterday at the Delta gate in Philadelphia, and the agents had to rely on x-ray machines to check the luggage yesterday.

On the plane, the Secret Service has Kennedy sit in the first row of the coach section. Agents then take over the seats directly behind and across the aisle from him.

But if the agents were anxious, the man they call "the protectee" was in a chipper, feisty mood.

Responding to a rousing recption from a cheering crowd of volunteers at his headquarters in Boston, Kennedy insisted he could come from behind and win once he gets Carter into a face-to-face debate.

"We're going to get him out of that Rose Garden sometime, and then we're going to ask him the questions he doesn't want to hear," Kennedy said.

And he made fun of Carter's suggestion that he could whip Kennedy in this year's campaign.

"We're going up to Maine and we're going to New Hampshire," Kennedy said, "and we're going all-1-1 the way to California and then we'll see who is going to whip whose what."

At the White House, deputy press secretary Ray Jenkins noted that Carter had just cancelled a scheduled appearance on Feb. 8 at a West Coast fund-raiser, because of his continued policy of declining public political appearances during the crisis in Iran.

"Under this circumstance, I don't think there is any chance for the president to engage in a political appearance by appearing with Kennedy at the consumer assembly in Washington)$," Jenkin said. "It would hardly sit well with our West Coast supporters for the president to turn them down and then engage in a partisan political debate."