Gary Hart had a book party in Georgetown last night, prompting guests to ask: How can a Democratic presidential candidate and U.S. senator find time to write a book, even one that will serve as a working paper for his political campaign? Did he really do it?

"It's reasonably safe to say I wrote most of it," said Hart. "And I edited all of it."

"All of the ideas are his own," said Maria Guarnaschelli, his editor at William Morrow. "What his staff did was flesh it out, turning position papers into something you could call a book. He's a very intelligent and literate guy."

"Substantially, it's his," said his press secretary, Kathy Bushkin. "There's probably not a sentence in there that he didn't work on."

The party for "A New Democracy--A Democratic Vision for the 1980's and Beyond" was at the home and in the garden of Mortimer B. Zuckerman, chairman of The Atlantic magazine. Zuckerman is a friend of Hart's and is supporting him for president. Zuckerman is also a real estate developer worth an estimated $150 million. He has houses in Boston, New York and East Hampton, but has recently acquired a fourth in Washington that he likes to fill with politicians, writers and people of influence.

"I figure that since I'm here so infrequently, when I can get a lot of people together, I feel like I'm using the place," Zuckerman said.

As it happened, he was a half-hour late for his own party, fresh from a squash victory at the University Club. "Fabulous game!" he said as he dashed into the front hall and threw his bag into the closet. He peeked into the living room, decorated with modern furniture, two huge ficus trees, a glass coffee table, brown monogrammed napkins and copies of the August issue of The Atlantic. Zuckerman flipped through it excitedly.

"The piece by Shirley Christian on Central America is absolutely wonderful. And the piece by Robert Weinberg is absolutely the best piece on cancer I have ever read. And remember Stephen Spender, who was here? He did a review on the book 'Modern Times' that is spectacular."

Hart was late for his party as well, hung up in a Capitol Hill meeting to decide the Democratic strategy for the MX missile debate today. This created an embarrassing situation for his aides, who kept eyeing the door nervously and saying, "We understand he's on his way."

Lee Hart, Hart's wife, was there a good hour ahead of her husband. The couple was reunited in January after a separation. A month later, Hart announced he was running for president.

"Working out our problems had nothing to do with the campaign," Lee Hart said last night. "Some people say it's so amazing how many divorces there are in public life, and I say it's amazing there aren't more . . . I'm sure we would never have gotten back together had I not enjoyed politics."

Hart's book is a text from the "new ideas" candidate that details his proposals on defense, the economy, arms control, etc. Among the chapters are "Shaping a Better Future," "Foundation for Growth" and "Perspective on a Changing World."

The new ideas candidate is still running far behind Ohio Sen. John Glenn and former vice president Walter Mondale. (Incidentally, a Mondale supporter was seen lurking at the party. "Don't say I'm here," he said.) Now the new ideas candidate claims to be running No. 3, ahead of California Sen. Alan Cranston, who also says he's No. 3. "He's more in debt than I am," said Hart.

But how much in debt is Hart?

"I don't know," he said. "I'm never good with check books. Ask Kathy."

"$244,000," said Bushkin. "We made $180 tonight from the sale of the books, but I don't think that will take us out of debt."

There were about 150 people at the party, most of them political reporters, Senate staff people, relics of the George McGovern presidential campaign (which Hart managed), a socialite or two, a couple of Kennedy operatives, a Jimmy Carter person or two. Among them: Frank Mankiewicz, until recently of National Public Radio; Sen. Max Baucus (D-Mont.); Jerry Rafshoon, Carter's former media adviser; and Rep. Morris K. Udall (D-Ariz.), who pulled out of the presidential race earlier this year and who said last night:

"It's a pretty good book. One of the things presidential campaigns ought to do is produce a book. This idea started out to get votes and all of that. But one of the byproducts is to force the candidate to sit down and decide where he stands."

By the time dusk was settling over Zuckerman's garden, neatly planted with petunias and the like, several Hart staff members were discussing their dinner plans.

"So did he write the book?" joked William Shore, Hart's political director.

"Didn't you write it?" said Kathy Bushkin, laughing. "You told me you did."