My guess - the "party of Lincoln" was a party of one

Lincoln. 

And he wasn't the abolitionist many paint him as being. 

Take a gander at his 1st Inaugural. 

Lincoln was devoted to preserving the Union, not freeing the slaves.  Check out when he issued the executive order known to posterity as the Emancipation Proclamation - not when war is declared, but immediately after the Union's first victory, at Antietam.  And it only applied to the states in rebellion. 

Yes, it was a stunning move & not just to stave off England or France coming to the aid of the Confederacy.  Lincoln's advisors were against the proclamation.  And he was proud of it - “I never, in my life, felt more certain that I was doing right, than I do in signing this paper.  If my name ever goes into history it will be for this act, and my whole soul is in it." 

Look at what happened after his death - the lack of mercy, the attempt to remove Lincoln's successor because he was trying to follow the martyred leader's wishes.  The ugly politics of the latter half of the century, politics that tied the Republican Party to big businesses & corporate big wigs.

Any semblance of the GOP to the "party of Lincoln" disappeared in the mid-'60s, with the passage of civil rights acts, then the introduction of the party's Southern Strategy.

The Washington Post writes that the party of Lincoln is dying.  They miss the real story - the party of Lincoln isn't just long gone, it was, at the highest political levels, a party of one