Like the growth of Irish, Italians, eastern Europeans, etc., Hispanics won't grow in population forever. In fact, we may be only a decade or two from the peak.
Huh?
Given that 95% of the population in the Americas outside of the US is Latinamerican and their societies will remain poorer than yours for the foreseeable future, how do you expect the Hispanic part of the US population to stop growing?
Trying to answer the question, I think that the Republicans will need to moderate their positions and roll back the Reagan-revolution and the current right wing excesses in order to stay competitive - this process will be troublesome and take at least a decade in the meantime Democrats will probably have a period as the dominant party in the 2020s.