For Mary Jo Peterson, the Senate Finance Committee's offices on Capitol Hill were a magic kingdom for almost eight years.

She met her husband Geoffrey there, when he was a dashing young staff assistant to Democratic Sen. Abraham Ribicoff. Later she introduced her handsome blond stepson to senators and others as a Senate page.

Now that is all but a memory for the slim, 35-year-old committee staff clerk, who is still reeling from this month's national election. In one fell swoop, three members of the Peterson household -- mother, father and 16-year-old Geoffrey Jr. -- lost their jobs, victims of a Republican landslide that hit both executive and legislative branches of government. Only son Jamie, who at the age of 10 is too young to hold a political job, was left unaffected by the election.

"It's just so depressing," says Mary Jo, who is worrying about where she and her husband, a $50,000-a-year deputy to an assistant secretary in the Department of Commerce, will find jobs to pay the rent on their Northwest Washington house and pay off debts than remain from her husband's unsuccessful race for a Connecticut congressional seat in 1976.

"I went to bed at 12:30 on election night, when we still had a Democratic Senate," says the $24,000-a-year staffer. "I guess maybe by then I knew what was coming, but I just didn't want to watch it."

Geoffrey Jr., a promising student who devours political statistics with all the vigor that many teen-agers direct toward baseball scords, says he and his Democratic Senate page friends have been "gloomy" ever since discovering that the election will cost them their $9,000-a-year jobs and berths in the Capitol Page School.

"The first time in 25 years that the Senate majority shifts, and it has to be when I'm working there," says Geoffrey Jr. ruefully. "It's like my father says: 'You live by the vote, you die by the vote.' And that's what happened."

Only 34-year-old Geoffrey Sr., a lawyer who helped generate Commerce tax policy and lobby administration positions in the Capitol, says he remains relatively unruffled by his family's personal election-day debacle. He hopes to get a lobbying job and remain in Washington.

I'm not sad or embittered that I have to go out there and find another job," he says, although he admits that post-election blues seem to making the Peterson family more irritable and tense. "I'm writing resumes, sending out notes to friends, talking to you," he said.

The Petersons' plight, first noted on National Public Radio, is typical, psychologists say, of what many soon-to-be-jobless Washington area residents are going through.

"A sudden loss is the most traumatic psychological experience a person can go through," says James W. Gottfurcht, a Los Angeles psychologist. "This election was a particularly good example. In one fell swoop, which the polls didn't even warn us about: chop, chop, chop. It was all cut off."

It would not be abnormal, psychologists say, for people who have gone through such an experience to feel anger, depression, anxiety and insecurity, as well as loss of identity, helplessness and insignificance. Such people might also find that they chronically feel exhausted, or that they are eating, sleeping or smoking too much.

For most in Washngton, they say, such symptoms should vanish before Ronald Reagan's inauguration with the help of some phpysical exercise, introspection and honest conversation with family members.

"It's not the end of us, for heaven's sake, but it's unsettling," says Mary Jo, who says she will miss the excitement of seeing celebrities like Elizabeth Taylor and Henry Kissinger, and occasionally appearing on television to the delight of her sister, a high school English teacher in Fort Lauderdale, Fla.

"This is something people are not going to get over for a long time," said Mary Jo.