"Drums! Beat! Drums!" supports Neely's position as to Whitman being a "Mystic Nationalist". Whitman more probably supported the Emancipation Proclamation because it is an act to reunite the Nation, but he may not have agreed with the actual document in itself. Even for President Lincoln the most important issue was to reinstate the Union, and not freeing slaves.
Whitman's earlier poems hint at him being a stronger proponent for the end of slavery, but he never portrays himself as a dedicated abolitonist during his life or during the Civil War. Whitman probably strongly supported President Lincoln because they shared similar ideals regarding the Union and the purpose of the war.
"Drums! Beat! Drums! is a fervent call to arms and declaration of pending victory for the Union, and he notes that all people should be a part of the union victory, as they will all here the drums beat. It is not necessarily speaking of the call to free the slaves.
Whitman's poem was one that draws on the emotions of readers and supporters of the Union. In the first stanza the drums "burst like a ruthless force". He calls for people to abandon their normal activities and devote themselves to the cause of the Union. Their way of life is being threated by the Confederate secession, and Whitman feels it should be defended by all.
In Whitman's stanza he calls for all people to play a role and fight for their liberties and a part of the Union. He urges people to not give sympathy to the forces, and for their passion for their country heard in the drums resound through all noises, even loud enough for the "trestles to shake the dead where they lie waiting the hearses".