"Nations do not die from invasion, they die from internal rottenness."

wisconsinhistory.org

     Jenkin Lloyd Jones, "Why love Russia," in Love for the battle-torn peoples:  sermon-studies . . . for the reinforcement of faith (Chicago:  Unity Publishing Company, 1916), 115.

     Though Abraham Lincoln gave voice to similar convictions, the very sentence in question was composed by the Unitarian minister Jenkin Lloyd Jones, not Lincoln. This is clear from the typesetting in context.  It is also clear from the fact that the noun "rottenness" does not occur anywhere in the standard 1953 Collected works of Abraham Lincoln online (which, however, does not appear to include the first (1974) and second (1990) supplements).  (The noun "invasion" occurs, but in no comparable context.)

     Also, the words of Lincoln with which Jones ends that same paragraph run not "If our republic ever dies, it will die from suicide, from degeneracy" but "As a nation of freemen, we must live through all time, or die by suicide."  Indeed, the noun "degeneracy" occurs only in the letter to Joshua F. Speed dated 24 August 1855:

I am not a Know-Nothing.  That is certain.  How could I be?  How can any one who abhors the oppression of negroes, be in favor of degrading classes of white people?  Our progress in degeneracy appears to me to be pretty rapid.  As a nation, we began by declaring that 'all men are created equal.'  We now practically read it 'all men are created equal, except negroes.'  When the Know-Nothings get control, it will read 'all men are created equal, except negroes, and foreigners, and catholics.'

     But from those words alone it should be obvious that, as I've already said, Lincoln gave voice to similar convictions.  Indeed, the entire thrust of the speech under discussion here, the "Address to the Young Men's Lyceum of Springfield, Illinois" delivered on 27 January 1838, is perfectly consistent with the summary provided by Jones, the instrument of the death by suicide that Lincoln warns against—"if it ever reach us, it must spring up amongst us.  It cannot come from abroad.  If destruction be our lot, we must ourselves be its author and finisher"—being, in that case, the "mobocratic spirit", i.e.

the increasing disregard for law which pervades the country; the growing disposition to substitute the wild and furious passions, in lieu of the sober judgement of Courts; and the worse than savage mobs, for the executive ministers of justice.

     Abraham Lincoln:  speeches and writings, 1832-1858:  speeches, letters, and miscellaneous writings, the Lincoln-Douglas debates, [edited and] annotated by Don E. Fehrenbacher (New York:  The Library of America, 1989), 31, 29;  Letter to Joshua F. Speed:  363 (360-363)).  (The Library of America edition is based on the 1953 edition edited by Basler (above), but corrects it where necessary.)

     Some low-hanging scholarship (articles) on the speech (most to least recent, in progress).  Note—on, at least potentially, the broader theme of internal degeneration—especially Wilson and Howe (which I haven't yet read):

• Ray, Angela G. "Learning Leadership: Lincoln at the Lyceum, 1838." Rhetoric & Public affairs 13, no. 3 (Fall 2010): 349–87.
• Fornieri, Joseph R. "Washington’s Farewell Address and Lincoln’s Lyceum Address." White House Studies 5, no. 3 (April 2005): 365–82.
• Wilson, Major L. "Lincoln on the Perpetuation of Republican Institutions: Whig and Republican Strategies." Journal of the Abraham Lincoln Association 16, no. 1 (January 1995): 15–25.
• Howe, Daniel Walker. "Why Abraham Lincoln Was a Whig." Journal of the Abraham Lincoln Association 16, no. 1 (January 1995): 27–38.
• Schwartz, Thomas F. "The Springfield Lyceums and Lincoln’s 1838 Speech." Illinois Historical Journal 83, no. 1 (January 1990): 45–49.
• Schmitz, Neil. "Murdered Mcintosh, Murdered Lovejoy: Abraham Lincoln and the Problem of Jacksonian Address." Arizona Quarterly 44, no. 3 (Spring 1988): 15–39.