Epiar's Curtis Dueck has a very interesting post on the ROI of the Presidential campaigns going on in the US. Curtis proposes to measure ROI in terms of awareness. It's an interesting metric to track and certainly plays a role in political campaigns. The last federal elections here featured, in my riding at least, candidates who were Marxist, Communist (apparently there's a difference there), Christian Democratic (as opposed to the other, undermocratic Christians who were running?) and more.
Their combined total was less than 1% of the vote. I'd say an important part of that is awareness. By contrast, the Green candidate got a few percentage points of the vote - again, awareness at work, imho.
But it's also not the be all, end all metric. Like I wrote back when the Liberals and PQ were off buying AdWords ads, there's a need for political marketing campaigns to define conversions. Some proposals I made were volunteers signed up, donations made and lawn-signs requested/distributed.