What does the President promise Knox in exchange for his support of Leffingwell in the novel, Advise and Consent?

The current president promises Knox in writing to support Knox as candidate for the presidency. This is done as a bribe in the attempt to get Knox to drop his opposition to Leffingwell. Knox shows the written offer to several other Senators as proof the President’s backhanded dealings and to prove that he has been offered such a bribe. The President’s bribe is unsuccessful as Knox ultimately returns the written offer to the President in an envelope.