Quoting Feynman from Chapter 2 of his book Six Easy Pieces (emphasis mine):
Suppose that we have two unlikes that attract each other, a plus and a minus, and that they stick very close together. Suppose that we have another charge some distance away. Would if feel any attraction? It would feel practically none.
On the other hand, if we get very close with the extra charge, attraction arises, because the repulsion of likes and attraction of unlikes will tend to bring unlikes closer together and push likes farther apart. Then the repulsion will be less than the attraction.
I get the first part. The two unlikes are so closely bonded that the distant charge can't affect them in any way. However, the second part is confusing. Why would the repulsion of the two bonded unlikes with respect to the third charge "tend" to be less than their attraction?