The implaceable economics of big-ticket political campaigning have now brought Sen. Edward M. Kennedy face-to-face with some painful decisions.

Should he pay people? Buy advertising? Or set up campaign offices in the big industrial states where he will mount his next major challenge to Jimmy Carter?

For the past month, in Maine and New Hampshire, Kennedy has enjoyed the luxury of large platoons of volunteers in his native New England and relatively cheap media.

But as he prepares to campaign in Illinois, New York and Pennsylvania, he is finding that everything there will be more expensive.

Accordingly, Kennedy has called a meeting of his senior campaign staff in Washington Thursday on his financial dilemma.

The meeting will probably also consider various proposals to reshape the national and local organizations. Kennedy's campaign manager, brother-in-law Stephen Smith, said Tuesday after Kennedy's loss in New Hampshire that "a considerable number of changes" in personnel and duties are likely.

The financial problem is neatly summed up in the person of Ellen Rothman, an ace political advance person who has been working for Kennedy since he entered the campaign as a heavy favorite last November.

Rothman, a 4-foot-11 bundle of energy, has logged thousands of air miles since mid-January, put in countless 20-hour days, and worn her nerves to a frazzle negotiating with hotel owners, telephone installers, bus renters and reporters all over Iowa and New England.

During that time, the campaign has given her a single "paycheck" totaling $40.

Rothman said Tuesday that she is still determined to see Kennedy win, but, "The problem is sometime "I've got to get home and earn some money."

Kennedy's campaign headquarters said yesterday that the entire 200-person staff would receive a full paycheck next week, but no one can say how regular future payments will be.

Rick Stearns, a high-ranking official in the Washington office, said contributions are back up to about $250,000 a week, furnishing enough to meet the full payroll March 1. A ceiling based on a $30,000 annual salary is in effect; those earning less will receive full pay.

Earlier financial difficulties had caused cancellation of the biweekly salary checks on Jan. 15 and reduction to half-pay for the Feb. 1 checks.

The candidate told reporters yesterday that he is acutely aware of the need to pay his staff. Yet a few minutes later he was talking about how much more expensive a campaign will be in big-media states than it was in New England.

"Out there," Kennedy said, "it's not like Iowa, Maine and New Hampshire where you're meeting people in the kitchen."

What will not change, the senator said, is the substance of the campaign, which now is focused fairly sharply on the nation's economic dilemma. In Alabama today, Kennedy built his speech around a text from Jimmy Carter himself. At every stop, he jabbed at Carter's Tuesday statement that current White House economic policies are "fine."

Kennedy, who reacted with aplomb Tuesday to his 11-point loss in New Hampshire, was tired and obviously bothered by a persistent cold when his entourage, reduced now to the capacity of four eight-seat planes, arrived in Alabama at noon today.

But he met with spring here on a bright, breezy, 60-degree day (exactly 59 degrees warmer than yesterday in Burlington, Vt.) that seemed to perk him up.

Campaigning with D.C. Del. Walter Fauntroy and nephew Robert F. Kennedy Jr., the senator shrugged off predictions that he is far, far behind Carter in the president's native South.

And he seemed unfazed, during a speech in Birmingham, by the sickest moment of his campaign to date.

About eight hecklers, who described themselves as conservative Republicans, were in the crowd and interrupted the candidate's speech with shouts and jeers. When Kennedy was explaining why he favors a ban on small handguns, he observed, as he always does in discussing the subject, "my family has been touched by violence." Some of the hecklers responded with cheers.

Wherever he went, Kennedy reiterated his determination to continue his campaign despite the New Hampshire result and despite his apparently bleak prospects in upcoming primaries.

"There's no question that the thing is moving, the economic message that the country won't take this inflation rate," he said. "That's our message, and we'll find more ways, better ways of getting it across."

In Montgomery tonight, Kennedy gave a powerful speech to a largely black audience, comparing his battle for liberal values to the Montgomery bus boycott of 1955 that effectively launched the civil rights movement.

But it was Fauntroy who brought down the house with a stirring endorsement of Kennedy and a blast at Carter. Fauntroy garnered so much applause when he ended his speech by singing "The Impossible Dream" that Kennedy felt obliged to stand up and remind the audience with a smile that he, not Fauntroy, is the candidate this year.