Cite the part of the constitution that allows secession. Be convincing.
I did some poking around and this is what I found:
The way to phrase the question is where in the Constitution is it said that they do not have the right to secede. All powers not given to congress are retained by the states. They do not need to be listed. But the powers of the Fed are listed.
The Federal government is based on the Declaration that states that the people have a right to dissolve the bonds between them ad the government:
"That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness"
Several states wrote in their acceptance of the Constitution specific statements that they were allowed to secede. Some have argued that if a few states had the right then all must.
The supreme court decision quoted by BigRob was a 5-3 decision so three of the supreme court justices thought the states had a right to secede. Those in favor of secession could argue that it was the 5 judges who were in error rather than the 3. An in the final decision all the justices wrote that there was a right to secede under certain circumstances: "through revolution, or through consent of the States"
(In the name of balance I will state that the argument against secession is that when the states chose to form a constitutional government rather than a Federation it is claimed that they all knew it would be permanent) Which is of course called into question because several of the states wrote that they expected the right to secede.
When Texas was accepted into the US it was while it was seceding from Mexico. Therefor the US accepted that Texas had the right to secede from Mexico.
It was commonly held before the civil war that the states did have the right to secede. The Supreme court decision saying that the right did not exist was in 1869 and the civil war was in 1861. Therefore while one might argue that states do not now have the right to secede it is a much harder case to make that at the time of the civil war they did not have the right.
(While I may be wrong about the legality of states rights to secede I personally think they should have the right and would add that if they do not it goes contrary to many of the foundational principles of our country.)
But, more back to the topic of the thread, if the states do have the right to secede that furthers the case that Lincoln was wrong. If they do not have the right then Lincoln was not wrong. It goes without saying that we decide if states have that right not based on how we wish Lincoln to be viewed but for other reasons.