They have been telling jokes about William J. Casey behind his back during the first weeks of his managership of the Reagan presidential campaign.

First, Casey misremembered the name of the campaign finance director.

Then, he misplaced the date of a key primary election.

Finally, he canceled a series of airplane charters for Reagan without telling the press about it, leaving three network crews in Atlanta while Reagan campaigned triumphantly in South Carolina and Florida.

But three weeks after Reagan dramatically ousted campaign director John P. Sears during their mutual moment of triumph in the New Hampshire primary, Casey is very much in charge of Ronald Reagan's campaign.

The New York attorney and onetime Securities and Exchange Commission chairman has meshed well with the entourage of Californians surrounding Reagan, as New York Attorney John N. Mitchell once did with the Californians around Richard Nixon.

Presiding over the firing of 100 Reagan aides, and the nonpayment of others, Casey has stemmed the financial hemorrhaging that threatened to drive the former California governor campaign into near-bankruptcy midway through the primaries. In so doing, Casey has won the solid, unqualified support of his candidate.

"In the campaign management business, you have a constituency of one," says a knowledgeable Reaganite. "And Casey has won the confidence of Gov. Reagan."

Casey also has gained the confidence of the Californian who is first among equals in the Reagan circle -- fellow lawyer Edwin Meese III. Meese, who didn't see eye-to-eye with Sears, says that Casey is "intelligent, exceptionally decisive and easy to get along with."

Others have used other words about Casey. During his years as a successful venture capitalist and book publisher, Casey was the target of a variety of lawsuits, including a successful one for plagiarism. Sen. Edward M. Kennedy once quipped that Casey was the "second most outrageous appointment as SEC chairman." The first being the senator's father, Joseph Kennedy.

At 67, Casey is only two years younger than his candidate and he has the wealth, legal experience and high-ranking connections that frequently impress political candidates.

But his origins were humble. After graduating from Fordham, Casey worked his way through night law school at St. John's while earning his living as a New York home-relief investigator. During World War II, Casey entered the Office of Strategic Services, the predecessor to the Central Intelligence Agency, and became chief of secret intelligence for the European theater.

William (Wild Bill) Donovan, the head of OSS, credited Casey with overseeing an important intelligence-gathering mission during the Battle of Bulge and wrote him in a letter: "You took up one of the heaviest loads which any of us had to carry at a time when the going was roughest, and you delivered brilliantly, forcefully and in good time."

After the war, Casey became a successful capitalist and a high-priced corporate lawyer. His government service, in addition to the SEC, included terms as undersecretary of state for economic affairs, and president of the Export-Import Bank. He is prominent in support of Catholic charities.

But does any of this experience qualify Casey to run a political campaign? Casey, at least, has no doubts. In a recent interview he emphasized that he would be in charge of the Reagan campaign's political strategy and traced his own experience back to his work for the 1940 Republican presidential campaign of Thomas E. Dewey, who lost the nomination that year to Wendell Willkie.

Certainly, Casey has long displayed an interest in the political process. He ran for Congress in 1966, losing the Republican nomination in a Long Island district. He was a friend and confidant of the late Leonard B. Hall, the legendary Republican chairman, and a member of Hall's New York and Washington law firms. He is given credit for rapid and accurate assembly of an issues book for Nixon's 1968 presidential campaign.

But for all of his high-level skills and friendships, Casey has never been a nuts-and-bolts political person. Some think he shows a lack of appreciation for the sensibilities of the campaign foot soldiers. A few in the press suspect that Casey harbors the pervasive mistrust of media characteristic of Nixon but only rarely of Reagan.

For all this, there are those who say that Casey was exactly what the troubled Reagan campaign needed.

"We needed an outsider to take charge, someone who could make decisions and hadn't been stained by all the infighting," says one Californian who has become a Casey convert. "Casey has imposed an objectivity on this campaign that was lacking before. He has also brought with him an understanding of international economic issues which will help to sharpen the candidate."

In the analysis of one familiar with inner workings of the Reagan operation, the replacement of Sears by Casey improved the campaign's management while diffusing strategic decisions. Political strategy is now largely a state-to-state affair in which heavy reliance is placed on field representatives recruited by Sears and deposed political director Charles Black -- such operatives as Roger Stone in New York and Connecticut, Gerald Carmen in Massachusetts and New Hampshire, Donald Totten in Illinois and Michigan and Lee Atwater in South Carolina.

The information and recommendations the field operatives provide are funneled into a strategy team which includes Casey, Meese, field director Andy Carter and pollster Richard Wirthlin.

Right now, everything is going well for Reagan, but there are inevitable moments of crisis ahead, and it is in these times that Casey will be tested.

"If the wisdom of chairman Casey includes an understanding of his own lack of knowledge about politics, this campaign is going to do just fine," says one who has long labored in the Reagan political vineyards."If not, well, I don't know. In politics, a little knowledge can be awfully dangerous and Bill Casey's knowledge of political campaigns for all his accomplishments, is still on the small side."