After Francis P. Blair returned to Washington from Richmond with a note from Jefferson Davis indicating a willingness to enter into negotiations, go here and here for background on Blair’s mission and his meeting with Davis, Lincoln had a decision to make. Refuse to enter into negotiations and that would anger both moderate Republicans and Democrats. Enter into negotiations, and both mainstream and radical Republicans would be dismayed. Lincoln hit upon a shrewd response. He would enter into negotiations, but he would couch his agreement in such terms as clearly to indicate no weakening in his resolve to preserve the Union:
You having shown me Mr Davis’ letter to you of the 12th , you may say to him that I have constantly been, am now and
shall continue ready to receive any agent when he or any other influential person now resisting the national authority
may informally send to me with the view of securing peace to the people of our one common country.
(Signed) A Lincoln
The ball was now back in the court of the Confederates. They could now enter into negotations, but Lincoln had made clear that preservation of the Union was not subject to such discussions.