They're not usurping authority, they're an equal half of a bicameral legislature. The entire point of such a structure is to either force compromise between the houses or shut down the government. The House isn't technically refusing to pass a budget, they have passed a budget, and the Senate disagreed. And of course the legislature has the power to modify previously-passed law. How else does one make new law without a discontinuous government? Also, I disagree with your use of the word "radical". "Radical" denotes political principles focused on altering social structures through revolutionary means and changing value systems in fundamental ways. The Republicans are focused on preserving historical social structures and value systems, through legislative means...