The most interesting event at last night's Democratic Congressional Dinner, where they raised $1.35 million, was when they lost Harold Washington. "Our New Democratic Mayor of Chicago!" boomed the introduction. "HAROLD WASHINGTON!"

The band struck up "Chicago," and everybody clapped for the city's first black mayor and the dinner's star attraction. It was hard to see where he was among 1,800 heads, but people figured he was there, somewhere.

He wasn't.

"He might have gone home to change," his aide, Helene Wallace, told a gaggle of reporters. "He was in Springfield today." She looked perplexed.

Meanwhile, the Democrats moved the program along so speedily that most people didn't even notice the mayor-elect was missing. Some even thought he was there. This caused some confusion among reporters and the dinner staff. Terry Michael, a Democratic National Committee press aide, was dispatched to the dinner floor to investigate. He came back empty-handed.

"I don't think he's here," he said. "I looked all over."

"I saw somebody stand up," said a photographer.

"I don't think it was Harold," said Michael.

"It was Harold."

Michael went back. No Harold.

"He's not down there, and I'm sure of it," he said. "I just talked to House Majority Leader Jim Wright and he said, 'If he's down here, I haven't seen him."

Washington finally arrived at 8:45. The television cameras swooped, bathing him in the kind of light that was usually reserved for Sen. Edward M. Kennedy (D-Mass.) when he used to say that he wasn't running for president. Then Washington got a second introduction, more applause and lots of back slaps on his way to the table. The Democrats, who weren't entirely enamored of a candidate who had income tax problems and a jail term behind him, sure love a winner.

"They might boo me at first," Washington said on his way into the ballroom. "And then they'll calm down. I'll stroke them."

Just think of how they'd have treated him if he'd lost. Pretty awful.

"Nooooo," said Kennedy, who came in a double-breasted tuxedo. "Do some politicians do that?"

The $1,000-a-head dinner is the annual fund-raiser for the Democratic House and Senate campaigns, and it is one of those things you have to go to if you want to be perceived as a part of the scene. Los Angeles Mayor Tom Bradley, House Speaker Thomas P. (Tip) O'Neill Jr., actress Lynda Carter and presidential candidates Sens. John Glenn of Ohio and Ernest F. (Fritz) Hollings of South Carolina were all there.

The House even cut off debate on the nuclear freeze resolution so everybody could get to the dinner on time. The guests quickly discovered it was just like the year before: crowded, noisy and behind schedule because of the interminable speeches. This year, though, everybody insisted it be short, so there was no major address--just singing and dancing.

Most of the singing was actually about the money they'd raised, a record amount. "People are beginning to realize that Reaganomics doesn't work," said Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee. "I think people are pretty up about the Democrats," said dinner chairman Anne Wexler.

In fact, Wexler had privately told friends that the real reason this was a record year was because she'd put the squeeze on the corporate people--the very people she'd often ushered in to see Jimmy Carter when she was White House assistant for public liasion, in charge of business and other interest groups. This is sometimes referred to as calling in the chits.

The other talk at the dinner was how the Site Selection Committee for the 1984 Democratic National Convention was going to vote today. They've seen San Francisco, Detroit, New York, Chicago and Washington.

"I hear San Francisco's going to be it," said Pamela Harriman, the Democratic fund-raiser who chaired the committee that made sure the site selection people had a good time when they were checking out Washington two weeks ago. "We put on a good show. But I think it was always going to be San Francisco."

To cover all bases, the DNC press office had already printed up a news release that said: "New York is Selected to be Site of 1984 Democratic National Convention" and another that said "Detroit is . . ." and another that said "Chicago is . . ." and so forth. The body of the releases was identical.

A lot of the people at the dinner were big contributors who flew in from across the country to hang around with the big names. Then there were those who really dressed. Like Bob Hannon, a cattle rancher from Amarillo, Tex. He wore a black leather and anaconda snakeskin dinner jacket.

"In New York, this would cost me $5,000," he said. "In Amarillo, it was only $1,200. But this is the last one I'm buying."