As stated, Lincoln's motives were to preserve the United States, and he did so, even at times acting illegally. The sectionalism that had been growing since the founding of the original colonies finally came to conflict in the 1860's; slavery was but a minor component to the differences between North and South. However, Lincoln brilliantly used the issue of slavery in the Emancipation Proclamation by "freeing all slaves in areas in rebellion against the United States" which didn't eliminate slavery, but allowed the Union Army to act as the liberator of the oppressed. By Lincoln actively stating the North's opposition to slavery (and this was done in 1863, during the middle of the war) he had also hoped to make this a foreign policy directive in keeping Great Britain out of the war on the Southern side. Since Britain had banished slavery in her confines in the 1830s, they could not openly recognize the Confederacy. Lincoln's gambit worked.
However, the Constitution specifies how areas may join the United States; it does not specify how areas may leave the United States. Had the Founders inserted that provision, the whole war may have been avoided. Lincoln did not preserve the Constitution; he preserved the Union by conquering the South.
I'm afraid this is too long (and interesting!) of a question to fully answer here... I'll give you the really short version:
His motive was to preserve the Union, not to end slavery. He said he would do whatever would save the Union whether that freed all the slaves, none of them or somewhere in between.
The effects of his actions were
- To cause the war
- To prolong the war (when Democrats would have negotiated a peace)
- To win the war (because his determination pushed the Union through to victory.
In my opinion he was very much a conservative. He wanted to remain true to the Union and to Constitution. That, to me, is conservative rather than revolutionary.
Great question, I hope this helps.