Nah, it doesn't matter whether this is constitutional or not, because it isn't actually intended to pass. It's just typical grandstanding. (I'm pretty sure that neither party actually wants to shorted copyright to 56 years)
Not only that, this can bring in extra revenue from sources that aren't voters (although those sources do have more lobbying power than voters).
Most works are actually abandoned by the end of 28 years, and this way someone else could pick them up.