Doing the right thing is usually divisive.
Presumably any state that had slave and joined the North would be allowed to keep its slaves to the detriment of the white workers.
Not "presumably" - according to the letter of the law they would. The Emancipation Proclamation had no effect on the slaveholders of Kentucky, Missouri, Delaware, Maryland or Washington DC.
Naturally the abolitionists wouldn't accept such an idea either.
Some abolitionists thought the EC wasn't good enough. The vast majority recognized it as an important step forward.
Lincoln's own attitude toward blacks demonstrates that the Proclamation wasn't based upon distaste for slavery as much as a war time measure against the South.
(1) Lincoln's "attitude toward blacks" changed over time. His attitudes and ideas in 1834 were not identical with his attitudes and ideas in 1864. Like most people, he learned quite a bit over the course of 30 years.
(2) Lincoln always had a distaste for slavery. He was one of the original freesoilers - one didn't need to have an enlightened view of black people in the 1830s to realize that slavery was a really, really bad idea.
(3) The EC was clearly a calculated move, but a calculated move that Lincoln also beleived was morally right. Lincoln quite openly told abolitionists that his main goal was perserving the Union and that he would agree to either maintaining slavery or abolishing slavery as long as the Union survived.
And Lincoln knew that once it was issued, the EC would prevent the UK and therefore France from entering the war on the side of the Confederacy.
Further given his treatment of any that spoke against his policies shows that divisions in the North cannot all be laid at the feet of any one group.
What "treatment" was that? Be specific.
And the draft riots show there was a full measure of racism in the North too
Of course there was.
Nor could it in those states, since it was a war measure that could only be imposed by presidential order on states in rebellion. But DC was a different matter. Congress passed, and Lincoln signed, a compensated emancipation act for the District on April 16, 1862, nine months before the Emancipation Proclamation.
“I have no purpose to introduce political and social equality between the white and the black races. There is a physical difference between the two, which in my judgment will probably forever forbid their living together upon the footing of perfect equality, and inasmuch as it becomes a necessity that there must be a difference, I, as well as Judge Douglas, am in favor of the race to which I belong, having the superior position. I have never said anything to the contrary, but I hold that notwithstanding all this, there is no reason in the world why the negro is not entitled to all the natural rights enumerated in the Declaration of Independence, the right to life, liberty and the pursuit of happiness. I hold that he is as much entitled to these as the white man. I agree with Judge Douglas he is not my equal in many respects—certainly n ot in color, perhaps not in moral or intellectual endowment. But in the right to eat the bread, without leave of anybody else, which his own hand earns, he is my equal and the equal of Judge Douglas, and the equal of every living man.”
Thus Lincoln's statement in 1858 is that blacks had a right to the fruitages of their labor like everyone else.
In other statements Lincoln supported laws that forbade inter-racial marriage, etc.
Lincoln was also prepared to accept a wholly slave U.S. or wholly free so his views were more pragmatic than moral.
What treatment of the opposition you say? Lincoln's order:
“EXECUTIVE MANSION, WASHINGTON, May 18, 1864. MAJOR-GENERAL JOHN A. DIX, Commanding at New York: Whereas there has been wickedly and traitorously printed and published this morning in the New York World and New York Journal of Commerce, newspapers printed and published in the city of New York, a false and spurious proclamation purporting to be signed by the President and to be countersigned by the Secretary of State, which publication is of a treasonable nature, designed to give aid and comfort to the enemies of the United States and to the rebels now at war against the Government and their aiders and abettors, you are therefore hereby commanded forthwith to arrest and imprison in any fort or military prison in your command, the editors, proprietors, and publishers of the aforesaid newspapers, and all such persons as, after public notice has been given of the falsehood of said publication, print and publish the same with intent to give aid and comfort to the enemy; and you will hold the persons so arrested in close custody until they can be brought to trial before a military commission for their offense. You will also take possession by military force of the printing establishments of the New York World and Journal of Commerce, and hold the same until further orders, and prohibit any further publication therefrom. A. LINCOLN.”
Then there was Clement Vallandigham whom Lincoln had sent to the South in a kind of internal exile.
You wrote:
“Some abolitionists thought the EC wasn't good enough. The vast majority recognized it as an important step forward.”
A step forward toward what? The Confederate states weren’t going to submit to the EP and it didn't apply to the Union states. Had the Confederacy won the EP would be moot and if the North won the EP would still be moot.