Two years ago, when the Chicago Merchandise Mart wanted to buy land in the District of Columbia, Mayor Marion Barry's administration helped it get a good deal. Thomas King, general manager of the Mart, calls Barry a longtime friend.

So when Barry asked for a chance to talk to Chicago's fence-sitting business community in support of Rep. Harold Washington's mayoral candidacy, King set up a private luncheon today at which Barry tried to convince 30 top Chicago executives that they should support Washington over Republican Bernard E. Epton in Tuesday's general election.

"There was some skepticism when it began, but I told them Harold is going to win and they better get on board," Barry said after the luncheon meeting.

The business community pumped millions of dollars into Mayor Jane M. Byrne's unsuccessful Democratic primary campaign and has since generally tried to steer clear of the bitter, hard-fought contest.

King took part in a brief attempt to launch a write-in campaign for Byrne, but backed away from the effort. He said today that he arranged the luncheon, held at the Merchants and Manufacturers Club, as a favor to Barry.

Barry helped arrange a $1.7 million write-down in the cost of a vacant warehouse in Southwest Washington that the D.C. Redevelopment Land Agency sold to the Merchandise Mart. The Mart has converted the property, at Fourth and D streets SW, to a $20 million Design Furnishing Center that is scheduled to open April 26. "I've enjoyed the mayor's business process," King said.

Some Chicago executives have reasoned that they have little to gain in an extremely close race by backing either Washington or Epton. "Frankly, we feel pretty content that whoever we get will be projobs, prorecovery and pro-expansion," said Sam Mitchell, president of the Chicago Association of Commerce and Industry. "We perceive both candidates as very probusiness."

Barry sought to convince the businessmen that Washington would be a prudent manager of the city's bureaucracy and finances.

Also today, Barry joined members of the American Federation of Government Employees in passing out campaign literature. Barry was joined by Bernard Demczuk, an AFGE official in Washington and one of his political allies.

"It's not too late--punch 8," Barry chanted through a loudspeaker outside the Social Security Administration, referring to the eighth position on the ballot, held by Washington.

Barry and the Washington campaign are paying Barry's expenses on the trip, except for the salaries of his two D.C. police bodyguards.

Barry has received a generally warm reception in Chicago. Daniel P. Harley, a U.S. Department of Labor official who lived in Washington between 1962 and 1966, said he felt Barry's appearances here were significant in demonstrating national unity among Democrats in trying to elect Chicago's first black mayor.

"This is not a local campaign," Harley said. "It's not just whether Harold will win. It's a national crusade."