In case you haven't heard, the Alfalfa Club is not a health-food organization and it's not exactly for fans of Spanky's gang, either. It is an all-male, purely social and socially pure group that meets once a year to eat, drink, poke fun at politics and nominate a member for U.S. president, to run on the Alfalfa ticket.

The Alfalfans are a formidable bunch of the nation's kings and kingmakers--including President Reagan, Vice President Bush, members of Congress, the Cabinet, the Supreme Court and chiefs of major corporations--and apparently they enjoy each other's company. Membership is limited to 150 and by invitation only (to fill vacancies left by deceased Alfalfans), but each member is allowed to bring three guests to the annual dinner. Generally, they "bring people who are going to make their futures," said one guest. "These are high rollers in business and politics."

"What I like about it is it's a nonmalicious atmosphere," said former secretary of state and former Alfalfa Club president Henry Kissinger.

"It has a good sense of humor," said another former secretary of state, Alexander Haig.

That understood, let it be known that the Alfalfa Club held its 70th anniversary dinner at the Capital Hilton Saturday night, as usual, behind closed doors and off-limits to women and press types. This year it nominated Washington attorney Edward Bennett Williams, minority owner of the Redskins, for U.S. president, and his acceptance speech was considered sensational, one of the best ever, according to Alfalfa veterans.

"I told him, 'I don't think the Redskins will choke Sunday, so don't you choke Saturday night,' " said attorney Robert Strauss, former chairman of the Democratic National Committee.

According to Alfalfans willing to share their knowledge, Williams' speech contained one delightful quip after another, roasting his "competitors" for the presidency in 1984, satirizing both political parties, presenting his own platform and, of course, cheering on the Redskins. For example:

* Sen. John Glenn (D-Ohio) and former Democratic California governor Jerry Brown would make good running mates, Williams said, because both had been in outer space--only Brown had significant reentry problems;

* Washington is the only city where you are never more than five miles from the scene of a crime;

* George Washington never told a lie but, Williams assured his audience, your candidate is not bound by this outmoded eccentricity;

* When the Chicago Democratic vote is counted last and the Republican candidate still wins, you know the Democrats are having problems;

* I'll get the rich and poor vote, Williams said, and make them think I'll protect them from each other;

* The way to organize a campaign on Capitol Hill is to find out which five people hate your guts and keep them away from the five who haven't made up their minds yet;

* And finally, to thunderous applause: The papers have been saying Miami's favored over the Redskins by three points, said Williams, but for those of you who are weak of heart, those are the same odds they gave General Custer.

The black-tie evening began at 6 with a reception, for which the president and vice president were whisked inside through the Hilton's back entrance, behind a curtained walkway. Women dressed in tuxedos escorted Alfalfa members (some of whom needed to be helped up the stairs), wearing club medallions on red, white and blue ribbons, through the Hilton's Art Deco lobby to the second-floor, private gathering. Among those entering:

White House Chief of Staff James Baker; White House Deputy Chief of Staff Michael Deaver; presidential counselor Edwin Meese; Chief Justice Warren Burger; Interior Secretary James Watt; outgoing Transportation Secretary Drew Lewis; Xerox Corp. chairman Peter McColough; U.S. special envoy to the Mideast Philip Habib; Charles Wick, director of the U.S. Information Agency; Federal Reserve Chairman Paul Volcker; editor Austin Kiplinger; and so on.

At 7:30, the Marine band played "Hail to the Chief" and Reagan entered the ballroom. Then the Joint Services Color Guard performed, and Senate Majority Leader Howard Baker (R-Tenn.), a former Alfalfa president, gave the opening remarks.

"During the patriotism of the moment," said Baker (according to one guest), referring to the fervor created by the Color Guard, "President Reagan leaned over and asked me whether we could pass the defense budget tonight."

Baker also pointed out that seven Alfalfa candidates had gone on to win the presidency, including Reagan, Ford, Nixon and Johnson.

The men dined on lobster, filet mignon and bombe glace' cardinale, and imbibed three kinds of wines. After dinner, President Reagan spoke. Among his reported jokes: "I see the Joint Chiefs of Staff are sitting in Dense Pack formation." The club also was entertained by Kerry McCarthy and Richard Stilwell of the Metropolitan Opera, and inducted new members, including the Capital Hilton's general manager, Frederick J. Kleisner.

The Alfalfa tradition, which began in 1913, contains pleasant memories for most, though some memories were more vivid than others Saturday night. When he arrived, Howard Baker recalled his first Alfalfa dinner in 1967. "That was my first year in the Senate. It was also the year Senator Dirksen, my father-in-law, fell and broke his hip, and he was supposed to speak. Republican administration veteran Bryce Harlow said, 'You gotta make that speech.' So that was my initiation. I had to make Ev Dirksen's speech, which of course, no one could do."

On the other hand, Labor Secretary Raymond Donovan, asked if he was a member, said, "Of what? The Touchdown Club?" Asked to recall a favorite moment at an Alfalfa dinner, Donovan said, "I remember Nancy Reagan's dance. That was memorable."

The first lady was part of the entertainment at the Gridiron Club's dinner also held at the Capital Hilton last year.

By midnight, 16th Street was jammed with parked limos, the Hilton lobby had filled with chauffeurs, and a few women in fur coats stood waiting to fetch their husbands. On their way out, club members and guests said they thought candidate Williams stood a good chance of winning, on the basis of his speech. "He gave one of his great jury orations," said Sen. Lloyd Bentsen (D-Tex.). "He probably prepared more for it than for a $1 million fee."