That laughing blond, dressed all in white, swinging on a white swing on the cover of Washingtonian magazine's June issue is none other than Marty Davis. And the congressman's wife looks every bit as good in her demure Laura Ashley as she did in her attention-grabbing exercise leotard . . .

James Gordon Liddy, the 23-year-old son of G. Gordon Liddy, convicted Watergate burglar, is undergoing training in the Navy's Aviation Officer Candidate School in Pensacola, Fla. A 1984 graduate of Fordham University, Liddy, explaining his decision to enter the Navy, said, "I knew I wanted to fly or be a doctor; I was brought up to have a lot of pride in my country and I feel this is my big chance to do a lot for my country" . . .

Before arriving here today in the royal family's Boeing 727, King Hussein of Jordan will attend the graduation of his 21-year-old son, Prince Faisal, who is receiving a bachelor of science degree from Brown University earlier in the day. It is the 50-year-old king's first trip to the United States since February 1984, and his wife Queen Noor and their 2-year-old daughter are with him. Hussein will be in Washington through Friday meeting with President Reagan, Vice President Bush, Secretary of State George Shultz, Secretary of Defense Caspar Weinberger and national security adviser Robert McFarlane for discussions that are expected to deal with a proposal of his and Palestine Liberation Organization leader Yasser Arafat to set up talks leading to the creation of a Palestinian state on land not held by Israel . . .

Former Massachusetts senator Paul Tsongas, who decided to quit Capitol Hill after learning he had cancer, told Boston Law School graduates that his illness, which is in remission, was not the only reason for getting out of government. One realization, he said, was that the yuppies are not the saviors of the '80s. "There is something wrong out there in yuppiedom. Excessive pursuit of materialism is very empty" . . .

Watch for the rush in Capitol Hill offices to read the latest edition of National Journal magazine. In it is listed the most and the least vulnerable senator up for relection in 1986. The most vulnerable, according to a variety of criteria, including percentage of votes received in the last election, President Reagan's election performances in their states and public opinion polls, are: Democrats Patrick J. Leahy, Gary Hart and Alan Cranston; Republicans Mack Mattingly, Robert Packwood and, tied for third, Jeremiah Denton, John P. East and Charles McC. Mathias. Those with the least to worry about are Jake Garn, Robert Dole, Daniel K. Inouye, Russell Long (retiring) and Wendell H. Ford . . .

Former automaker John DeLorean, acquitted last August on cocaine conspiracy charges, said he plans to show in his forthcoming book that the government's case against him was "pure fabrication."

Speaking at the American Booksellers Association convention in San Francisco yesterday, DeLorean said he was a "sick and strange guy" at the time of his arrest, but cooperated with alleged drug dealers for fear he would be killed . . .

Washington seems to be a town where magazines survive and proliferate. And there may be another one coming. James Rice, associate publisher of The Washington Weekly and former publisher of The Washington Monthly, has announced plans for a fortnightly magazine tentatively titled Washington. Looking for the usual "New Yorker-style audience," Rice says, "There is a demand here for a magazine that fully reflects the intellectual curiosity and cosmopolitan life style of the people who live and work in the Nation's Capital." He hopes to have the first issue out next spring . . .

Just when you thought it was safe to go out on the town feeling secure that the fund-raising auction season was over for the summer, look again. On Saturday night the Corcoran School of Art is holding a black-tie, $50-a-person "Saturday Night A-Live with Art Buchwald." And guess who's the guest auctioneer, ably assisted by Michael Grogan, a professional auctioneer from Sotheby's. Some of the items on the block will include the works of Sam Gilliam, Rockne Krebs and Mindy Weisel, as well as the photographs of former senator Howard H. Baker . . .

Beth Newburger, former Washington Post marketing manager and New Republic magazine advertising director, has been named publisher of The Washington Woman magazine . . .

Veteran entertainer Billy Eckstine opens at Charlie's tonight, and the Pittsburgh native will receive a gold medallion making him an honorary member of the Pittsburghers of Washington, D.C. Inc., a predominantly black organization that raises funds for scholarships to deserving students from metropolitan Pittsburgh who attend Washington area colleges . . .