A severe cash shortage has forced a drastic curtailment of the Ronald Reagan presidential campaign at the very time the candidate appears to be riding high politically.

Reagan's financial problems could become more acute in the weeks ahead, and George Bush, at least, may be in position to take advantage of that.

In a move to conserve rapidly dwindling resources, the Reagan campaign abruptly canceled three airplane charters through the South, leaving members of Reagan's staff and the traveling press stranded in Columbia while the candidate took off on a small jet to Atlanta.

"We used the charters earlier when we could have flown commercial," said Rep. Carroll A. Campbell Jr. (R-S.C.), Reagan's South Carolina chairman. "Now that we desperately need the charters, we don't have them."

Campbell and South Carolina campaign director Lee Atwater had planned a triumphant rally as a final event to give what Campbell called "emotional charge" to the campaign. Instead, Reagan settled for an undramatic bus tour of rural South Carolina, where Sen. Strom Thurmond (R-S.C.) has been campaigning hard for John B. Connally.

Reagan's financial problems, some of which have been evident since the firing of campaign manager John P. Sears on the day of the New Hampshire primary, could become crucial in the months ahead if Reagan is not able to lock up the Republican nomination quickly. George Bush's campaign, for example, has spent about $9 million so far, and will be able to outspend Reagan in the late primaries in May and early June.

Reagan's abrupt change in plans resulted from two factors. His campaign spent more than $12 million last year and in the first two months of 1980 so lavish that he had exhausted at least two thirds of the federally permitted ceiling of $17.6 million for the long nominating process. At the same time, Reagan's South Carolina campaign was bumping up against the $470,000 state spending ceiling allowed by federal law.

Campbell said that in South Carolina three days of television advertising had been canceled and the radio budget slashed by 40 percent. But a mailing of 63,000 postcards was sent out today to pro-Reagan voters, and Campbell predicted that the former California governor would win a convincing victory in Saturday's Republican primary here.

The charter cancellations were ordered by Reagan campaign director William Casey at a meeting Wednesday. Edwin Meese III, one of Reagan's principal aides, said the action was necessary because of a lack of money to pay for the charters.

Meese said that the cash flow problem was severe because news organizations, the Secret Service and other users of services had been slow in paying their bills. The slow cash flow has forced the Reagan campaign headquarters in Los Angeles to curtail expenses and cancel plans for several hirings, including that of San Diego executive Ed Gray as press secretary.

Though Reagan is looking forward to victories in South Carolina, and in Alabama, Georgia and Flordia Tuesday, the euphoria that prevailed in his campaign after a smashing New Hampshire primary victory 10 days ago is largely gone.

Aides and advance men wondered aloud how long their jobs would last or if they would be paid. Among those who are in it for the long haul, there is concern that Reagan's money problems may provide added encouragement to induce Gerald R. Ford into the race.

"We're in great shape in the South and we ought to win in Illinois [March 18]," said one Reagan aide. "But if anything happens, anywhere, we simply have no margin of error."

At a news conference here, Reagan said he was sorry there wasn't better notice for the reporters about travel arrangements and acknowledged deficiencies in his own organization. But he said that the chief problem was not with his campaign, but with a federal election law that placed unfair and unrealistic limitations on candidates.

Strategists in opposing camps calculate that Reagan will spend another $3 million by April 1, leaving him about $3 million to spend for the 25 primaries in April, May and June and at the Republican National Convention in July. According to an official of the now defunct campaign of Howard H. Baker, Jr., Reagan has been advised to save at least $1 million for the convention, which would leave $2 million to finance campaigns in those primaries. "I don't see how he does that," this Baker man said.

The Reagan camp's reply is that Reagan won't need significant amounts of money for the later primaries, because he'll have the nomination sewn up before they begin.

Reagan's problems will be over if none of the other Republicans can stay in the race after March. Bush campaign aides say he can and will do so. s

"We're the only one" who can stay in and stay active beyond March, Bush campaign director James Baker said in an interview. He predicted that Reagan won't be able to match Bush's spending in late April and May.

Like Reagan, John B. Connally, who built his campaign on the presumption that it would be well-financed, has also spent nearly $12 million, and he is broke. Connally's campaign seems destined to collapse unless he can win big in South Carolina or Florida, both unlikely prospects, according to the polls.

John B. Anderson, who revived his struggling campaign in Massachusetts and Vermont this week, is in no danger of running up against the federal election law's spending limits, but his campaign may be too poor to fight actively in the next round of primaries.

So far Anderson has spent about one 10th as much as Reagan and Connally (roughly $1.4 million). His campaign is now raising money at a rate of about $200,000 a week, a sharp improvement over its earlier performance, and enough to finance a respectable media campaign in Anderson's home state of Illinois.

The Illinois primary on March 18 is the next one Anderson plans to contest seriously.

So far the Bush campaign has raised $8 million from the public, has received $1.4 million in federal matching funds, and has spent $9 million, or at least one-fourth less than Reagan. The Federal Election Commission owes the Bush campaign an additional $2.3 million in matching funds, according to Bush's spokesman, Peter Teeley, and contributions from the public are coming in regularly.

"We are reaching the point where we won't have to spend a great deal on raising money," said James Baker, Bush's campaign director. Bush will need $2.7 million in public contributions to reach the spending limit of $17.6 million (counting federal matching funds), and Baker predicted that this goal would be reached by May.

If it is, and the prediction depends on Bush staying politically alive through all the March and April primaries, Bush will have several million dollars for late primaries when Reagan could be broke.

Bush's relatively healthy financial condition is the result of effective exploitation of his early caucus victories. In February, after his unexpected Iowa victory, Bush raised $2.5 million, Baker said.

Using these figures to argue his case, Baker said no other Republican in the field could stay with Reagan through the rest of the campaign. He is probably right, unless Anderson can do so well in his native Illinois that he attracts substantial new donations.

How Reagan and Connally could each have spent $12 million already, and Howard Baker $4 million, is a lesson in the new political realities. Part of it was pure wastefulness induced by early wealth, no doubt, but much of those millions went to direct mail campaigns (intended to raise money) and expensive television advertising in the early caucus and primary states.

Reagan at one point had a paid staff of 350, a huge army to have to pay. Until Connally's staff -- once as large as 390 people -- recently went on "volunteer status," that campaign's payroll was $95,000 a week. By comparison, the Anderson payroll is $45,000 a month.