All those big-name Democrats gathering at the Madison Hotel tonight may well be outshone by a truly hot auction item -- a six-seat box at the Kentucky Derby May 4. About 300 guests, paying $1,000 or more each, will be there to hobnob with those glittery Democrats at the "Senate Majority '86" fund-raising dinner given by Pamela C. Harriman's PAC, Democrats for the 80's.

Some 30 senators have said they will attend the black-tie evening, including the newly elected Albert Gore Jr., Tom Harkin, John Kerry, John D. Rockefeller and Paul Simon, as well as some of those whose seats are up in 1986: Sens. Dale Bumpers, Alan Cranston, Alan Dixon, Christopher Dodd, Wendell Ford, John Glenn, Gary Hart, Ernest Hollings, Daniel Inouye, Patrick Leahy and Russell Long.

Sen. Wendell Ford will have the evening's big moment when he auctions off the Kentucky Derby box on the Club House Finish Line donated by Louisville Mayor Harvey Sloane and his wife, Kathy. It comes complete with invitations to attend some of Derby Weekend's most exciting parties. That one should bring in the bucks. Even before the auction begins, there already is an opening bid of $6,000.