I reviewed the most comprehensive selection of Abraham Lincoln’s speeches and writings and there is no evidence he said he should wait until after the election to tender his nominee for Supreme Court Justice.1 Lincoln had to wait to present his nomination since congress was not in session again until after the election. The 38th Congress was in session Dec 7, 1863 to July 4, 1864 and did not resume duties until Dec 5, 1864 (Dec 4, 1864 – March 3, 1865). Chief Justice Roger B. Taney died on Oct 12, 1864. Lincoln nominated Salmon P. Chase as Chief Justice on December 6, 1864, the day after Congress resumed its duties and the Senate was in session.
It seems incomprehensible to me that Abraham Lincoln, a man who dedicated and even gave his life for the cause of equality of all individuals would choose not to replace the man who oversaw the Dred Scott decision, knowing he had a 66% majority in the Senate and his other four Supreme Court nominees had been swiftly confirmed. To me the real explanation is that he never said it was better to wait, but he had to wait since congress was not in session.
1. Lincoln, Speeches & Writings, 1859-1865. Vol. I 10/1989 Library of America. Abraham Lincoln. Edited by Don E. Fehrenbacher. Trade ISBN 0-940450-63-1