It's less to do with the fact that they're accepting money from causes they are sympathetic toward and more related to the fact that they are now under an oblig

It's less to do with the fact that they're accepting money from causes they are sympathetic toward and more related to the fact that they are now under an obligation to continue supporting the causes they've received money from or stop receiving donations, against the wishes of their constituents. See, for example, the tax bill of 2017.