The downward spiral here isn’t tracing the decline of the Republican party but the descent of Congress, which, from the Affordable Care Act to the new tax-cut bill, has shown itself incapable of proceeding according to regular order, of conducting its business in a fashion befitting the legislature of the most powerful nation in the history of human affairs, and of forging bipartisan compromises — which are desirable not because bipartisanship and compromise are virtuous but because achieving broad political buy-in is the only way to produce stable and long-lasting policy settlements. The Affordable Care Act began coming undone the second it was signed; this tax plan, created in much the same way, may very well suffer the same fate. Whatever the corporate tax rate is when Trump signs the tax bill, it is unlikely that it will stay there for very long if Democrats come back into the majority in Congress. And who believes that Republican congressional majorities are destined to be eternal?
The Republicans are very lucky that the only practical alternative to them at the moment is the Democrats. The Democrats are lucky in precisely the same way.