To begin with, it would have been almost certainly illegal/unconstitutional. The 5th Amendment grants that the government may not deprive anyone of property without just compensation, a fact many Southerners used to argue that any anti-slavery measures would have been unconstitutional without being paid (although most would have not desired the payment in lieu of just keeping slaves in bondage). Lincoln was certainly a pro-active president who often blurred the line between what was exactly legal from a Presidential standpoint, but ending slavery single handedly was not within his reach, no matter how long his arms (and they were actually long). A president cannot simply declare property to no longer be legal despite his and much of the nation's views on it. Abolitionists and anti-slavery men and women well knew that to end slavery it must be done legally. Yet the Supreme Court in Dred Scot v Sandford (1857) had ruled the Missouri Compromise unconstitutional, effectively signalling the Federal government had no ability to restrict slavery in territories. Slavery was legal in 15 states in 1860, and a far stricter Federal Law required return of runaway slaves, a law already rooted in the Constitution.

It must be understood as well that the Emancipation Proclamation was a carefully crafted executive order that freed slaves only in the areas in rebellion. The legal argument is that a force rebelling against the United States forfeits their property, including cannons, horses, ammunition, shoes, medicine, tents, powder, etc. If Union soldiers captured Rebel cannons, there was nothing legal barring the confiscation and use thereof. Lincoln added slaves to this list of "contraband." (Many of the black Union soldiers would sometimes be referred to as "contraband units.") The Confederacy heavily utilized slave labor in their few factories, the fields harvesting cotton to provide money and potential aid from England, or digging trenches, hauling lumber, or tending to soldiers themselves as body servants. By rebelling, the Rebel areas (note: Lincoln does NOT see them as seceded, non-US states, but states who are in rebellion) forfeit their slaves. The document did not include areas already seized by the Union (therefore no longer rebelling), or those areas that had not joined the Confederacy. This included many counties in VA, LA, all of TN, MD, KY, DE, and MO. The Proclamation is basically saying, "since you are using slaves to rebel, we reserve the right to seize them and use them as we wish, which includes freeing and arming them." Some 180,000 black soldiers eventually served in the war for the Union side. Even THIS document did not end slavery entirely. It would take the 13th Amendment in 1865 to fully end it everywhere.

Secondly, had Lincoln made attempt to put forward such a Proclamation in April 1861, two things were likely to occur. One was that Kentucky and probably Missouri would have seceded immediately along with the four upper South states that did following Lincoln's call for 75,000 volunteers after Sumter. These states, Kentucky especially, wished to be in the Union AND maintain slavery. Such an act would have given them prime motive to secede (part of the reason the Proclamation was written to not include Kentucky). The second thing that would likely have occurred, would be that the courts would have immediately ruled against such an act as far exceeding Presidential authority. Even Lincoln himself rolled back efforts of early Union commanders to free slaves. And when the Proclamation was put forward, many in his cabinet and certainly thousands in the nation were shocked by it.

Which also brings to question the fact that Lincoln immediately attempting to free slaves was exactly what the Confederacy was afraid of. They seceded based solely on the idea that a Republican president meant the end of slavery, and they abandoned their country to secure those slaves. For Lincoln to play right into their hands would ultimately have put the US, and the Republican party especially, in a precarious and untrustworthy condition. It is for this reason Lincoln, in his inaugural, tried in vain to assure Southern states his intent was not to end slavery:

I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.

Those who nominated and elected me did so with full knowledge that I had made this and many similar declarations and had never recanted them;

Although Lincoln was against slavery practically his entire life, and would do more than any one man every did in ending in America, he also understood the political situation, and the law. Slavery was legal for the time being, but secession was not. Restoring the Union took precedence always, but when the time was right, Lincoln was able to put into place a legal measure to begin the ending of slavery, a measure he unfortunately would not live to see come full circle.