When it came to political plans for the post-war nation, he was strictly opposed to most Republican-proposed measures. Republicans proposed the Thirteenth Amendment to the Constitution in 1865, which would abolish slavery, and Randall spoke against it. Claiming opposition to slavery, Randall said his objections stemmed instead from a belief that the amendment was "a beginning of changes in the Constitution and the forerunner of usurpation". After Andrew Johnson became president following Lincoln's assassination, Randall came to support Johnson's policies for Reconstruction of the defeated South, which were more lenient than those of the Republican majority in Congress. In 1867, the Republicans proposed requiring an ironclad oath from all Southerners wishing to vote, hold office, or practice law in federal courts, making them swear they had never borne arms against the United States. Randall led a 16-hour filibuster against the measure; in spite of his efforts, it passed.