on a state by state basis, you might find this map from Latino Decisions somewhat useful.
You can click on the state to get details.
The data for predicting the top line outcome of the state may be somewhat dated, since the map was last updated on October 28, and things have been moving in the direction of Obama ever since.
What is key is the expected percentage of the vote Latino Decisions believes will be Latino - usually at least a few points over that shown in most public polls - and how they expect the Latino vote to split.
Thus, in the new NBC/Marist/WSJ poll of Florida, we find the poll expects the electorate in FL to be 14% Latino, whereas Latino Decisions expects the share to be 17.4%. Further, Latino Decision says only 31% of those voters will break Republican.
Thus if you add 17.4%, and take a rough 2-1 break in favor of Democrats, the additional 3.4% would add something over 1% to Obama's margin in the state, if Latino Decisions' modeling is correct.
Here are the figures for some other battleground states
State % Latino % of Latino Republican
CO 12.1 21
NV 14.7 18
AZ 20.0 16
WI 2.2 24
IA 2.4 27
MI 2.0 24
OH 1.5 24
PA 2.8 24
NH 1.5 24
VA 2.3 23
NC 2.9 24
If things are very close, the understating even of the small proportions in VA and NC COULD make a difference.
And understating of the substantial share and distribution in CO and AZ are worth considering as one evaluates both where the electoral votes go and in the case of AZ how the Senate race will turn out.