Mr. Kozak is now a party and witness in a class-action lawsuit that has meandered through the Federal courts for 15 years and charges that Mr. Trump owes $4 million to the union welfare funds for the work the Poles performed. Filed in 1983, the suit has been bogged down by a torrent of motions and appeals of judicial decisions and by the deaths of a judge, a lawyer, the original lead plaintiffs, an important witness and two of Mr. Trump's co-defendants.