And that is possible.
From PC World:
Here's the trailer for the documentary, UnCounted The Movie.
Diebold Election Systems Inc. voting machines are not secure enough to guarantee a trustworthy election, and an attacker with access to a single machine could disrupt or change the outcome of an election using viruses, according to a review of Diebold's source code.
"The software contains serious design flaws that have led directly to specific vulnerabilities that attackers could exploit to affect election outcomes," read the University of California at Berkeley report, commissioned by the California Secretary of State as part of a two-month "top-to-bottom" review of electronic voting systems certified for use in California.(more)