Not if, when.

"When we take control of the Senate . . .

". . . when a Democrat steps onto the inaugural platform to take that oath of office . . ."

The tone was "upbeat" and "optimistic," in the words of many senators and would-be senators, at last night's annual dinner of Democrats for the 80's, the powerhouse political action committee founded by Pamela Harriman.

It was like a mini-Democratic Convention.

There was a lot of campaigning, networking, interfacing. Maine Sen. George Mitchell, the emcee, introduced Sen. John Glenn as "one of our great heroes from Ohio," and everyone cheered. D.C. Mayor Marion Barry was put before the crowd as "a good Democrat who's served the nation's capital with distinction," and everyone clapped.

But most of all, there was a lot of talk about retaking the Senate. The feeling last night at the Ritz-Carlton was that, with the 34 Senate seats that are up for election this year, it will happen. And that this year will be only the beginning.

"Next year's dinner will be held in honor of the new Democratic majority in the United States Senate," said chairman Harriman, who went on to outline a three-step program: to retain the House majority, to regain a Senate majority, and to put a Democrat in the White House in 1988.

Sen. Edward Kennedy, head of another Democratic PAC, the Fund forsw, a Democratic Majority, showed up to shake a few hands.

"I think that the Republicans have at least 10 times the financial backing and resources that the Democratic members of the Congress and the Senate have," said Kennedy. "Democrats have always depended on the people, but they need a certain amount of resources to get their messages across. Tonight will make that possible."

And on a lighter note, was he ready for this weekend's marriage of his niece Maria Shriver to Arnold Schwarzenegger?

"As much as I can be," he said, and slipped off through the crowd of tuxedos.

It really was the fundraiser for Democratic senators, and a number of congressmen who are trying to move into the Upper Chamber -- John Breaux (Louisiana), Tom Daschle (South Dakota), Wyche Fowler Jr. (Georgia), Jim Jones (Oklahoma), Harry Reid (Nevada), Richard Shelby (Alabama) and Tim Wirth (Colorado) -- also showed up. The dinner, which attracted 381 candidates and supporters, raised about $350,000 for the cause.

"I can't raise enough money in my state," said Breaux, who, according to the PAC, is running in one of this year's toughest and most significant races, against Rep. W. Henson Moore for the seat being vacated by Russell Long. "We've got the highest unemployment in the country," he said. "It's so depressed by Reagan's programs."

Minutes before the dinner started, Colorado's Gary Hart, who is retiring from the Senate, perhaps to seek the 1988 presidential nomination, was asked whether there were any Republicans he particularly hoped would lose their seats.

"The list is too long," he said.

Most members of the Loyal Opposition said they did not think the Libya bombing would be a campaign topic.

Said Kennedy: "It's not a partisan issue."

Said Breaux: "People in Louisiana feel the president did the right thing. I said the same thing."

Said Ohio Sen. Howard Metzenbaum: "When our nation is involved in a confrontation, we rally round the president. After the dust settles, people start to ask the questions."

Although Libya has caused some in Washington to be concerned about terrorist attacks, Janet Howard, the PAC's executive director, said the group had arranged for nothing beyond the usual security measures.

"We had two people try to crash the party," she said, "and we took care of it." The two she referred to were picketers who earlier had carried signs for Lyndon LaRouche, two of whose followers recently won Democratic primary races in Illinois. The two picketers said they thought LaRouche was to be one of the speakers at the dinner.

In addition to rallying speeches by Harriman, Mitchell, dinner cochairman Sharon Percy Rockefeller and former Democratic National Committee chairman Bob Strauss, the evening offered entertainment that everyone hoped provided an omen. Kitty Carlisle Hart, longtime friend of Pamela Harriman, sang a tune she said she was performing in a new Woody Allen movie due out next January. The song began, "I'll Be Seeing You in All the Old Familiar Places . . . "