When that is the case, you have an agency problem . Senator Mitch McConnell of Kentucky is the main opponent of campaign finance reform, and the issue has been used against him in elections several times without success. If the public cared enough, it could vote out representatives who did not do its will on campaign finance. In fact, majorities that are large, intense and persistent enough can probably assure the passage of anything they want. Delegates think of themselves as perfect agents, doing exactly the bidding of those who sent them, their principals.
Trustees think of themselves as selected to use their own judgment.
See the FDR and World War II case on pp. 385-387 for an illustration. But trustees have to be elected too, so perhaps pure trusteeship is impossible in a democracy, so long as members want to be re-elected. You also know that it is possible that the public will is not coherent, and that even representatives who wish to be delegates must make up their own minds on what they want. Bicameralism makes changes to the status quo more difficult than they would be with a single chamber.