Why did Abraham Lincoln need to wait for the Union Army to win a battle before issuing the Emancipation Proclamation?

Abraham Lincoln did not officially have to wait for a Union victory to issue the Emancipation Proclamation.  By this, I mean that there was no law or anything like that that said the proclamation could only be issued after a Union victory.  Instead, he felt that he needed to wait for political reasons.  He felt that he needed to wait so that it would not look like he was issuing the proclamation out of desperation.

At the beginning of the war, most people in the North believed that the war was a war to bring the Union back together, not necessarily a war to end slavery.  The Northerners did not want slavery to spread because they did not want to have to compete with slaves for land and work.  However, they generally did not care if slavery continued to exist in the South.  Politically speaking, it was easier for Lincoln to portray the war as a war against secession, not a war against slavery.

By late 1862, however, Lincoln had come to believe that slavery had to be abolished after the war.  For this reason, he drafted the Emancipation Proclamation.  However, the Union army had been doing very badly at the time when he drafted it.  He did not want to release it at that point because he felt that it would look like an act of desperation.  Lincoln feared that it would make the North and his government look weak.  He wanted to wait until the Union was doing better so that it would look like he was freeing the slaves because it was the right thing to do, not because he felt that he had to do it in order to keep from losing the war.