They are opposed to a corporate tax rate cut that isn't offset by some tax increase elsewhere.
Why? One of the ideas behind lowering taxes is to incentivize spending, which in corporate means expanding and, ideally, in the end, more taxes being paid overall, while the tax burden remains the same or lower.
The consensus view on the American left is that the idea you outlined is incorrect. Or, more precisely, that there theoretically exists some tax rate where lowering it would produce more overall revenue, but that empirically we appear to have already been well below that rate before the cut.
Why? One of the ideas behind lowering taxes is to incentivize spending, which in corporate means expanding and, ideally, in the end, more taxes being paid overall, while the tax burden remains the same or lower.