What prompted congress to propose the 14th amendment?

Republicans grew concerned over the increase the Thirteenth Amendment would create in the congressional representation of the Democratic-dominated Southern States. Because the population of freed slaves would now be counted for determining congressional representation, rather than three-fifths, the Southern States would dramatically increase their power in the population-based House of Representatives, regardless of whether the former slaves were allowed to vote. Republicans began looking for a way to offset this advantage, either by protecting and attracting votes of former slaves, or at least by discouraging their disenfranchisement.