Lincoln in 1864 was running against Maj. Gen. George B. (Little Mac) McClellan. McClellan was running as a Democrat on a Peace Platform, which in effect would have made McClellan the "Disunion" candidate. To contrast himself against McClellan, Lincoln would have to run on his record as being for the preservation of the Union, and therefore a "War" candadate. This is what made Lincoln think that he would NOT be re-elected in the summer of 1864 and why the Casuality reports from that summers campaign were misstated in Grants reports.

It doing this I don't think that Lincoln was "Jumping Ship" from the boat that brought him to the White House by calling himself a Unionist rather than a Republican. Lincoln was certainly a "Unionist" and even close to being what I would concider an "Unconditional Unionist", as I understand the usage of these terms, as far as Lincoln's political objectives are concerned.