During the 2008 presidential election we submitted letters to the editor at 100 major U.S. newspapers as part of a field experiment to test whether interest in the letter depended on which candidate the letter supported. We find, contrary to what charges of a liberal media bias would suggest, that newspapers expressed more interest in pro-McCain letters than pro-Obama letters. Further, we find that papers were most likely to be interested in letters supporting the candidate they did not endorse, a result that is consistent with the idea that editors seem to be using their gate keeping powers to allow dissenting opinions to be heard.
That is from a newly published paper by Daniel Butler and Emily Schofield. An ungated version is here.