This strikes me as a far more important consideration than the loss of revenue or potential harm being discussed in the other topics.

Blacking out Google's front page will reach 38 Million (or 12% of the population) [1]. Compare that to the effort involved in more traditional political advertising--The largest TV audience is around 27 Million [2] (And drops by 50% to the most popular non-football program). There's also the question of how invasive the ads are. I'd bet a notice on the Google homepage getting in your way every time you do a search will have far more impact than newspaper ad that you skip over once you realize it isn't an article.

We know there's a level of discomfort concerning business's influence on politics. There was fallout over the supreme court decision in Citizens United v. FEC--the issue is being brought up again [3].

Google has a lot of power over the internet and a lot of knowledge of what people do on-line. Most of us can overlook that since we don't see a personal impact (except every few months a site vanishes from Google's results and the owner's blog post shows up on HN). Though they are getting attention from congressional inquiries and lawyers seeking to kill opposing sites.

Consider the fallout if Google went ahead. You have people uneasy with how businesses can effect politics from just the abstract idea of donations being shown a very concrete example. You have politicians with the power to regulate Google being shown that Google can be a danger to them.

I don't see how they could risk losing their perceived neutrality and harmlessness.

[1] Using the statistics from http://google.com.hypestat.com/ for a rough estimate: 129M unique visitors daily, 30% from the United States, that's 38.7M or 12% of the population.

[2] http://www.nielsen.com/us/en/insights/top10s/television.html

[3] http://online.wsj.com/article/SB1000142405297020436810457713...