Why did Union leaders believe the issuance of the Emancipation Proclamation would harm the South's ability to wage war?

By offering amnesty to Confederate deserters, the Confederate Army would rapidly decrease in size.
By encouraging slaves to flee North, the Confederacy would lose a key source of labor for the war effort.
By freeing all slaves held in the United States, the Union hoped to attract more soldiers to its cause.
By establishing abolition as a goal of the Union, the Union was able to gain the support of Great Britain.

The second option is the most accurate answer. Union leaders believed that the issuance of the Emancipation Proclamation would harm the South's ability to wage war because it would encourage slaves to flee to the North. This would result in the Confederacy losing a significant source of labor for its war effort, weakening its ability to sustain itself economically and militarily.