A new Pew Research Center poll indicates that the majority of Americans think that being an Atheist is a liability for a presidential candidate but that this majority has been declining over time. Per the article (which contains a wealth of statistical information):
The conventional wisdom in American politics has long been that someone who is not religious cannot be elected president of the United States. Most Americans have consistently said that it is important to them that the president have strong religious beliefs. And a new Pew Research Center survey finds that being an atheist remains one of the biggest liabilities that a presidential candidate can have; fully half of American adults say they would be less likely to vote for a hypothetical presidential candidate who does not believe in God, while just 6% say they would be more likely to vote for a nonbeliever. On the other hand, the share of American adults who say they would be less likely to vote for an atheist candidate has been declining over time….The new survey confirms that being an atheist continues to be one of the biggest perceived shortcomings a hypothetical presidential candidate could have, with 51% of adults saying they would be less likely to vote for a presidential candidate who does not believe in God. Indeed, in the eyes of the public, being a nonbeliever remains a bigger drawback than having had an extramarital affair (37% say they would be less likely to support a candidate who had been unfaithful), having had personal financial troubles (41% say they would be less likely to support a candidate who had had financial struggles), or having used marijuana in the past (20% would be less likely to support a former pot smoker)....At the same time, the new survey also finds that the share of Americans who have reservations about voting for an atheist president has been declining over time. As recently as 2007, more than six-in-ten Americans said they would be less likely to support an atheist presidential candidate, while just 51% express this view today. Over this period, the share who say a candidate’s lack of belief would not be a factor in how they vote has been growing.
It looks like they have trouble to define and find a moral person....
I think it's an historical hangover that atheism is demonic. It's been indoctrinated into the mind that human moral instinct comes from divine command; hence the distrust of anyone not blessed with the gift of gods grace.