This particular scary canary in a coalmine is known as presidential "undervotes"--ballots cast without recording a choice for the highest office in the land.
In this clip, UNCOUNTED focuses on instances of undervoting in two battleground states in 2004--New Mexico and Pennsylvania. New Mexico had a particularly large problem, where presidential undervote rates of 25% were reported in Democratic-leaning Hispanic and American Indian precincts. New Mexico had the nation's highest presidential undervote rate--21,084. (George W. Bush won that state by less than 6,000 votes.)
2008 might have a lot of things in common with 2004. Now we'll know what to look for if undervotes are one of them.