In practice, anything it wants. In my not so expert legal opinion? Anything it wants, through the proper process if not already granted that power. Let the house and senate pass an amendment, let the states ratify it. Or the opposite in which the house, senate, or states say *expletive deleted*ck off. This is supposed to be the built in check to limit the federal governments authority. Fat load of good that it's done us to one degree or another pretty much since inception.
Which is why I find the "living document/modern interpretist" bit so amusing. The process to change it is right in there. Changing to meet modern needs is a built in feature, with proper checks and balances, which is the problem to them. The process is based on the notion that the federal government has limited authority, and that to get more they have to go begging to the people (via their representatives) and to the states. The vast majority of washington and most people think it's the other way around, the notion that it isn't elicits a reaction ranging from perplexed to violently offended.