On a day when Ronald Reagan was entitled to savor his triumph in New Hampshire, his presidential campaign instead confronted painful necessities -- reorganizing the staff and slashing the budget.

Reagan said today his campaign already has spent $12 million, a pace that would have him bumping up against the federal spending ceiling sometime in April. Under federal election laws, a candidate who accepts federal matching funds may spend no more than $17.6 million trying to win the nomination.

Staff spending will be slahsed from about $500,000 a month to about $250,000, campaign sources said. This means that about half of the 275 persons now working for the Reagan organization will be laid off. Furthermore, there is serious discussion of closing the Washington office and running the campaign from Los Angeles, as the candidate prefers.

"We did have excessive staff in the early primary states and we're going to have to make some cutbacks," Reagan said in an interview en route to Columbia, S.C. "There may be some good come out of this for us because now that people know we're short of money, we're getting lots of calls for volunteers."

In one of the swiftest script changes of either his acting or political careers, Reagan on Tuesday replaced the strong-man direction of campaign manager John Sears with a collective leadership strongly flavored with Californians. j

While New York attorney William J. Casey was named new campaign director, Reagan said leadership and strategy will be shared among several advisers, as it was in the most successful period of his California governorship.

And the man who played the central role in the Reagan administration at the time, San Diego attorney Edwin Meese III, has emerged as the first-among-equals on the Reagan team.

Sources in the Reagan campaign said one of the several incidents leading to the departure of Sears, political director Charles Black and press secretary Jim Lake was their attempt to diminish Meese's influence with Reagan.

Reagan has long trusted and liked Meese, an affable, 48-year-old constitutional lawyer with an irreverent sense of humor. When a reporter last summer asked Reagan whom he would rely on if he faced a crisis, Reagan replied without hesitation: "Ed Meese."

Another San Diegan who will play a significant role on the new team is pollster Richard Wirthlin, who provided the survey data for both Reagan presidential campaigns. Wirthlin is a respected Republican pollster whose surveys more often than not were filtered to the candidate through Sears. Now, Wirthlin will be an issues adviser in his own right.

Reagan said he expected Wirthlin to take up much of the slack left by the departure of Sears, whom Reagan praised as a strategist. He added, however, that Wirthlin has "extensive polling experience for national candidates and knows a lot about politics."

The new team also will rely on a cadre of congressional supporters, heavily centered in the West and South, of whom Sen. Paul Laxalt of Nevada is the most influential. Laxalt, chairman of the Reagan for President Committee, played a key role in the decisions leading to Tuesday's election-eve staff shakeup.

At press conferences today in Andover, Mass., and Burlington, Vt., Reagan continued to insist he had not fired Sears, saying he had offered to keep Sears as his strategist if Sears would relinquish day-to-day control of the campaiagn to Casey. Sears chose to resign rather than accept a diminished role.

While Casey and Meese flew together to Reagan's Los Angeles headquarters today, the waves from the New Hampshire shakeup continued to widen like ripples in a pool. There were several resignations in Washington, including Nick Ruwe, the scheduling director who had performed the same function for Richard Nixon's 1968 campaign. Like many others who are leaving, Ruwe is a confidant of Sears who had brought him into the campaign.

By midday, the number of resignations or dismissals totaled 12, with other departures expected.

Both Casey, who headed the Securities and Exchange Commission during the Nixon administration, and Meese have reputations as efficient managers, and because of the money crunch it is likely to take all their skills to keep the campaign from facing the embarrassing alternative of halting activities in the spring or violating the law.

In restructuring his campaign forces today, Reagan reached back to some of the Californians he had depended upon during his eight years as governor or in his 1976 presidential campaign. But Reagan today downplayed the California connections of the new team, saying, "We don't want to create a California mafia."

While no announcements have been made, it was learned that Reagan expects to name Ed Gray of San Diego, once his gubernatorial press secretary and an ally of Meese as his campaign press secretary in Lake's place.

Reagan will also give a larger role to Martin Anderson, a former Nixon administration economist and author who started out as Reagan's issues adviser in November. Then, after a dispute with Sears, he returned to his post at Stanford University's Hoover Institution under circumstances which have never been fully explained.

Anderson has been advising Reagan occasionally since, but it is anticipated that his role will be expanded now. He will be in Columbia, S.C., Thursday, to brief Reagan for a debate that night with Bush and other Republican presidential candidates.

Another Californian mentioned as a probable member of the new Reagan team is Paul Shirley, director of scheduling in 1976 and a likely candidate to replace Ruwe.

Longtime Reagan associate Michael K. Deaver, who also left the campaign after a dispute with Sears, will not rejoin the Reagan organization directly, according to reports. However, Deaver's associate Peter Hannaford is serving as interim press secretary and will continue working in the campaign.

The void left by Black will be filled largely by Andy Carter, director of field operations and an advocate of grass-roots organization with close ties to southern and western conservatives. These conservatives were the strongest critics of Sears and the chief movers for his removal.

There was a bittersweet quality to the Reagan campaign today, a reflection of the friendships and emotional tensions between staff aides and reporters in a long campaign. Some aides expressed pleasure at Sears' departure; others wondered plaiantively where their friends of yesterday had gone.

Reporters, most of whom liked the departed aides, joked among themselves about other firings, including the shakeup in the Nixon administration's Interior Department where an executive, leaving for lunch advised: "If my boss calls, get his name."

And one reporter remembered, as an afterthought, that Lake had won a $140 pool for predicting the New Hampshire results.

"We'll have to go get the money together and send it to him in a check," a reporter said. "But first, we'll have to find out where to send it."