It was mostly a District affair, except that the invitation-only fundraiser for Sen. Patrick J. Leahy (D-Vt.) was held two weeks ago in the stately suburban Maryland home of multimillionaire developer Nathan Landow.

For those who received the engraved, "You are cordially invited" summons to Landow's Bethesda estate, it was a chance to give $500 apiece to the chairman of the Senate Appropriations subcommittee on the District.

It was jointly sponsored by Landow, D.C. Democratic Chairman Robert B. Washington Jr., real estate lobbyist Morton Funger and corporate lobbyist Thomas Hale Boggs Jr., son of the late House Majority Whip.

Accompanying each invitation was a warm little note from Landow that said, "Pat Leahy is chairman of the Senate District Committee. He is a very important friend in the Senate for all of us to have. I know you will find the evening a most pleasant and informative one." It was signed, "Best regards, Nate."

With the Democratic convention just around the corner, there was plenty of political fodder as the contributors bent elbows and chatted at the cocktail and dinner reception. But there were noteworthy items other than those on the menu.

For instance, the ink was barely dry on the agreement that gave Washington a no-investment equity interest in Landow's $30 million Metro Center office development in downtown D.C. Both men have been big fund-raisers for President Carter's reelection campaign and have become friends through their work on the Democratic National Committee.

Meanwhile, Landow certainly had cause to feel the part of the gracious host: he had just closed the deal to sell the 21-story Promenade apartments in Bethesda for $54 million to a Chicago condominium converter. With an existing mortgage of only $23 million, the deal reportedly yielded $30 million in cash.