Nothing improves Washington's night life like an election. New Orleans has the Mardi Gras and Louisville has the Kentucky Derby, but here, everyone knows the horses.

Last night -- from the beef bourguignon Ronald and Nancy Reagan served to friends in the White House family dining room, to the chicken hash at Averell and Pamela Harriman's, to the wine and brie offered to lawyers and young congressional aides at a rowhouse on Capitol Hill -- official Washington turned out to monitor its friends, enemies and future. Admittedly, some people went bowling. But in an off-year election that was especially dicey, this normally sober town stayed up late and turned electric.

Typically, the Democrats cooed over the House seats they gained, while the Republicans maintained it wasn't much of a victory. Everyone was tense. "So how are your nerves?" Peatsy Hollings, the wife of Sen. Fritz Hollings (D-S.C.), asked Carol Williams, a longtime Democratic supporter.

Replied Williams: "I am absolutely a wreck." Eminence in Georgetown

"We won New Jersey!" Pamela Harriman screamed to Clark Clifford, throwing her arms in the air and grabbing his shoulders.

"That's great!" said Clifford, the Washington eminence. "That's just really great!"

Things were burbling along early at the Harrimans in Georgetown, where the Democratic party poobahs dropped by before heading off to make their official network pronouncements. In fact, they could have made them there; CBS was doing live interviews with celebrity guests from a terrace by the pool. "There's a saucer on the roof," marveled Pie Friendly, a Harriman faithful.

There were also mini-TV sets on glass tables in the garden, a yellow-striped tent in the backyard, Dan Rather on a large screen on the porch and an extensive roll call of important Democrats. Everybody was in a good mood, smelling blood. Pamela Harriman, an aristocrat who caused snickers when she first set up a political action committee, now has gained respect because of the money she's raised for Democrats. This was the number-one place to be seen last night.

Former vice president Walter Mondale came early. "I feel good," he said cautiously, "although I never did expect this election to be a blow-out." Sen. Gary Hart (D-Colo.) arrived a little later and, asked when he was going to announce for president, said: "Tomorrow morning. 8 a.m. My office." Peatsy Hollings was there, too, at first without her husband. "He's working the crowd," she explained. "I think he's working the chauffeurs out on the street first."

Charles Manatt, chairman of the Democratic National Committee, was in fine spirits. "It looks like a miracle," he said. "You may recall from a year ago that the Republicans planned to take the House, three governorships and five Senate seats. This year is a miracle." Then he announced that election night was "like having a baby."

A number of Carter Democrats surfaced under the Picasso and van Gogh, among them Anne Wexler, Stuart Eizenstat, Gretchen Poston, Lloyd Cutler and Bob Strauss. Clark Clifford, meanwhile, announced that the Democrats would gain 30 seats in the House and three in the Senate.

"In order to preserve the possibility that I might be regarded as a true prophet, I'm not breaking the figures down," he said. "And if I'm wrong, then I'll be just an overly enthusiastic Democrat." Ruminating, he added: "There is more interest and more excitement tonight than in any other off-year I can remember -- with the possible exception of 1946. It was the first off-year election since Roosevelt had died . . ." Wipe-Out Time -

Things weren't funereal, but almost, at Richard Viguerie's party for Republicans at the Mayflower Hotel. Guests quietly grazed under silver candelabra as a pianist played "Witchcraft," while Viguerie, the conservative fund-raiser who always complains about the moderates in the White House, complained anew.

"Everybody in the country understands that the White House ran a weak, defensive campaign," he said. "Except the White House. You need some sophisticated political people around the president, and at the top level, you've got nobody. We told them, 'If you make it a referendum on Reaganomics, it's going to be wipe-out time.' "

Still, Viguerie was taking a philosophical--or perhaps religious--approach to the whole evening. "I told a dozen of my friends earlier tonight, 'If we win or lose, it's not our doing.' God's will is going to be done. He's got His plan."

Meanwhile, you could spot Interior Secretary James Watt over by the buffet. His approach to the evening's mess was the long-range view. He predicted: "1984 will be a landslide." Informed that Viguerie was blaming the White House for the election, Watt pondered this, then announced:

"I'm going to have a sandwich." Happy Days

At the Capital Hilton, the ballroom was shaking with "Happy Days Are Here Again" as Manatt, Pamela Harriman, Kentucky Gov. John Y. Brown and assorted others stood before 2,000 Democrats who were singing, cheering, drinking and dancing. "We're clearly going over 20 seats," DNC Executive Director Gene Eidenberg said at 8:45 p.m. Most of the crowd, made up of young Hill staffers and campaign people, stood wide-eyed and delighted.

"It's better than Georgetown," said Elizabeth Veanus, a 21-year-old DNC intern who likes to go to Winston's on M Street and who was wearing a "Stay the Curse" button.

Drowned out by all the noise were Roger Mudd and Tom Brokaw, who loomed larger than life from a giant screen in the corner. But there were more television watchers than dancers, even when what was widely regarded as a terrific rock band struck up "Fame." During the early part of the evening, a few danced in their sensible pumps or daringly took off their jackets, but generally, they looked as if they felt a little silly.

Allison Stak, a young director of media planning for a public relations firm, described it this way: "Reminds me of a bar mitzvah." Other Places Around Town

The most elegant (although probably not the most fun) place to be last night was the White House, where the Reagans had a buffet dinner for friends and key advisers. Among them: Ed Meese, James Baker, Mike Deaver, William Clark, Sen. Paul Laxalt (R-Nev.), Vice President George Bush, Secretary of State George Shultz, Republican National Committee Chairman Richard Richards and strategist Stu Spencer. Everybody had dinner served from an American mahogany sideboard that Daniel Webster once carved with his initials.

Over in the West Wing, mid-level staffers manned an "election central" in the Roosevelt Room, which was painted salmon some time ago and which all the men complain about as being too sissy. The large table was taken out and replaced by small ones covered with yellow cloths. "Looks like a dinner party," said deputy press secretary Larry Speakes who, with aides David Gergen, Ken Duberstein, Lee Atwater and Rich Williamson was at Table No. 1. Everybody else manned phones. Down the hall in the White House press office, the lower-level staffers had Chinese carry-out.

And in Georgetown, at former Reagan aide Joe Canzeri's place, White House speechwriters, East Wing staffers and assorted secretaries dined on beer, wine, apple cider and Little Tavern hamburgers. Canzeri, opting for a "We Were Expecting This Approach," insisted everyone felt just fine. One Candidate's Schedule

Mondale, an undeclared presidential candidate who's been campaigning hard, began his evening at the Harrimans. Then he went to a party at his law firm, headed over to CBS for an interview, went to an AFL-CIO party at union headquarters on 16th Street, then, at midnight, went back to his office. "Not bad, huh?" he said.

On his way out of the AFL-CIO building, he walked by two security guards who seemed to be enjoying the action. Candidate Mondale couldn't resist. "Good night!" he said, heartily grabbing the hand of guard Frank Applewhite. "Good night!" And then, to guard Sam Smith: "Good to see you!" A wave, and he was gone. The Die-Hards

By midnight, the parties were winding down. Mayor Marion Barry had long before arrived at the Capital Hilton with an entourage like that of a presidential candidate. At the row house on Capitol Hill, 28-year-old lobbyist Neil Simon and his wife Becky Burr, a 27-year-old real estate developer, were waiting with some die-hard friends for the California returns. At the Capitol Hill Club, the somber crowd of Republican Senate and House campaign committee people was heading home to bed. And at Canzeri's, just a few hamburgers were left. "The worst thing that could happen to the Democrats is if they won the Senate," he said, flicking a fat cigar. "Then everything will be their problem."

"I've heard that," said Dick Moe, a Mondale intimate. "But given the choice, I'd rather win the Senate."

Or as Gene Eidenberg put it: "I'm from the old school. Winning's better than losing."