Lincoln on Secession:

Any people anywhere, being inclined and having the power, have the right to rise up and shake off the existing government and to form one that suits them better. Nor is this right confined to cases in which the people of an existing government may choose to exercise it. Any portion of such people that can, may make their own of such territory as they inhabit. More than this, a majority of any portion of such people may revolutionize, putting down a minority intermingling with or near them who oppose their movement.  

Lincoln on the floor of Congress, 13 January 1848
Congressional Globe, Appendix
1st Session 30th Congress, page 94