I believe you are thinking of Buchanan.

Historian Elbert Smith explains:

"After admitting that certain grievances would justify secession, Buchanan then insisted that secession was unconstitutional. The founding fathers had never intended any such right, and the principle had been clearly 'refuted by the conclusive arguments of General Jackson' in 1833. The Union 'was intended to be perpetual and not to be annulled at the pleasure of any one of the conflicting parties.'

". . . The president's rather brilliant constitutional analysis was tempered, however, by a paragraph that appeared to justify the right of secession as long as it was called revolution instead of an inherent constitutional right. [This touches on my earlier distinguishing between revolution and secession in Lincoln's speech.]

". . . Meanwhile, said Buchanan, the president must obey his oath of office, but how and to what extent? The machinery for enforcing federal laws in South Carolina had already disappeared, and only Congress could amend existing laws to restore federal authority in that state. . . . The federal government, Buchanan insisted, had no power either to recognize secession or to coerce any state to remain in the Union. The Constitution delegated no such power to either the president or Congress."

Smith, The Presidency of James Buchanan (Lawrence: The University Press of Kansas, 1975), 150-151.

Check up on your source and see if the author was actually talking about Buchanan, not Lincoln. If he/she was talking about Lincoln, please provide more info, such as a source or quote from Lincoln.