The pressures were many and they were intense. The war was well underway by the summer of 1862, and Lincoln was under political pressure from the abolitionist lobby, which had been united, strengthened and emboldened by the war. They wanted Lincoln to act decisively towards emancipation. What was the President worried about, they wondered, offending the South? The pro-slavery Democrats were gone from Congress, seceded along with their countrymen, so many abolitionist northerners lobbied Lincoln for immediate and full emancipation.
Lincoln had strategic interests to consider as well. The war was going badly for the Union, and emancipation, he worried, might seem an act of desperation, hence his decision to wait until after the Union “victory” at Antietam to issue the Emancipation Proclamation. He also hoped it would forestall the possibility of cotton-starved Britain's intervention on the side of the South.
Most importantly though, would be the effect the EP would have on the border states, particularly Maryland. It was already under virtual martial law, and the President had suspended habeus corpus there and ignored the warnings of the Supreme Court in order to do so, which led to his careful wording in the proclamation not to free any slaves in states still loyal to the Union. The text and timing of the Emancipation Proclamation was, in my opinion, just one example of his political genius.