Contrary to popular belief, Abraham Lincoln did not actually liberate any slaves through the Emancipation Proclamation. The Emancipation Proclamation only released slaves that belonged to the rebellious states that had formed the Confederacy. It did not release any slaves in the few Border States that still remained loyal to the Union. This was a reasonably astute political move. If he had liberated the slaves in the states that were still loyal to the Union, chances are that the remaining slave states would have seceded and joined the war on behalf of the Confederacy.
At this point in time Lincoln technically had no jurisdiction over the Confederate states so the Emancipation Proclamation he issued, freeing all the Confederate slaves was useless and it did not even free a single slave in the Union. However, it did serve to act as a moral boost for the North. They now had the competitive edge in the war. The Civil War had become a battle with a moral cause. This also helped secure the war for the north as it eliminated any chance of foreign intervention.
Lincoln was truly a great man. He had what it took to keep the United States together. However, in doing so, he infringed upon many Constitutional rights. For an extended period of time he suspended the privilege of Habeas Corpus. Lincoln also took over several newspapers and prohibited others from continuing distribution, which is a direct infringement of the rights stated in the First Amendment of the Constitution.
Lincoln greatly expanded the powers of the Executive branch during his time in office. It may very well be argued that his actions were justified and reasonable. However, in most cases such as this, when any President greatly expands his powers, the other branches of government must act to take them back once they are no longer necessary for the President to do his job effectively. This most likely would have resulted in the impeachment of Lincoln during his second term. It may not be beyond reason to assume that his untimely death could have saved his image from an impending political assualt and preserved him as one of our Nations greatest heroes.