As Hispanics integrate, they will be lumped more into the existing racial categories. I'm Cuban, but look indistinguishable from an Italian or Greek person. Plus, like immigration from Ireland, Italy, Germany, etc, immigration from Latin America will eventually slow, and may be doing so already.
I think this is a good point and may be one of the reasons that Hispanic/Latino is not actually considered a race but an ethnicity. I think that Hispanic Americans are likely to begin assimilating into the culture in the same way earlier waves of European immigrants were able to. Many people who identify as being Hispanic have virtually no outward physical characteristics to distinguish them as being such.
While I believe that Hispanics will integrate more and already have, the source of immigration - Mexico - means that many of these immigrants have disconnected from their original homeland. Also looking at Hispanics in places like New Mexico there is still a sense of "otherness." Because of their skin color, Hispanics still are treated as "others" rather than "Whites," and likely will remain that way for longer than other immigrant groups.
What's more likely to change voting patterns in my opinion is the increasing wealth of Hispanics so that they are more moderate voters like Asian-Americans.
