How American Non-Atheists view Atheists
"The fool says in his heart, “There is no God.” They are corrupt, they do abominable deeds, there is none who does good."
- — Psalm 14
"atheists are at the top of the list of groups that Americans find problematic"
- — Penny Edgell et al., 
Minnesota study, 2006
American adults have less tolerance towards atheists than towards people who belong to a different Christian sect or a different faith. A 2006 study performed by the University of Minnesota found atheists to be the least trusted identifiable group in the United States, as well as the group parents would be least accepting of their children to marry. 
Gervais study, 2014
A study of Americans, including theists and atheists, found that they judged various immoral acts to be representative of atheists but not of other social and religious groups. 
Gallup has been polling people on whether they would refuse to vote for "a generally well-qualified person for president" if they were an atheist for over 40 years; here are the historical data: (from the "Gallup Polls & Other Surveys" link, below)
The religious affiliations in America are changing rapidly. Between 1990 and 2001 the proportion of United States citizens without religious affiliations apparently rose from 8% to 14% while adults believing in Christianity dropped by 1% per year in that time. (2001 ARIS study; ReligiousTolerance.Org entry on it) If this drop continues over a generation the United States will change dramatically. Not all without religious affiliations are Atheists, agnostics, or freethinkers, but many are.
People in the United States overestimate how many atheists there are in the country, with the average response being 40% and the actual proportion being 16%. 
- How Non-Atheists view Atheists from ReligiousTolerance.Org
- Gallup Polls & Other Surveys on American Attitudes Towards Atheists / Over 40 Years of Research Show Atheists Are Despised, Distrusted by Austin Cline
- 10 facts about atheists, Pew Research, June 1, 2016
- Obama Signs New Law Giving Protection To Atheists