edit: perhaps I made a mistake to mention Fisher. The linked documents always identify Benjamin Burton as both a Republican and a large (for Delaware) slaveholder but Fisher is only mentioned as a slaveholder in the NYT op-ed
Here are a few corroborating sources: Slavery and Freedom in Delaware, 1639-1865 by former University of Delaware professor William H. Williams, and NYT piece by author of "Lincoln and Race" professor Richard Striner.
I stumbled upon this fairly randomly and was not able to turn up any additional information.
edit: I'm not certain Fisher was a slave owner but Benjamin Burton, the largest slave owner in Delaware, is identified as a Republican in every source I found.
Was George P. Fisher a slave owner? He was the point man that Lincoln chose to act on compensated emancipation because he was the only Delaware Congressman. That blog post claims he was one of the largest slave owners in the state, but that seems unlikely. Fisher was a lawyer, not a planter like Benjamin Burton. That blog post also erroneously calls Burton a Congressman, which he certainly was not.
William Burton, a relation of Benjamin's, was Governor of Delaware at the time but he was a Democrat.