For the first time ever, the US no longer has a majority of Protestants as the number of people with no religious affiliation rises, a study has found.
The Pew report found only 48% of adults identified themselves as Protestants, down from 53% five years ago.
The long-expected decline was pinned to a rise in those claiming no religion - about 20% of Americans, the study said.
There are no Protestants on the Republican presidential ticket for the first time this year.
Nor are there any Protestant Supreme Court justices.
The Pew Forum on Religion and Public Life said the number of people with no religious affiliation was up five percentage points from 15% in the last five years.
The category includes atheists, as well as people who believe in God or who identify themselves as "spiritual" but not "religious".
The study concluded most of the respondents were not seeking new ties within another religious institution.
One third of adults under the age of 30, but just 9% of those older than 65 claimed to have no religious identity.
Correspondents say the trend away from religion could have political implications.
The report found that those with no religious affiliation are much more likely to support abortion rights and gay marriage than the rest of the American public.
They also tend to be registered as Democratic voters, so this bloc could soon become as important to Democrats as evangelicals are to Republicans, the study said.
Pew found that Americans without religious affiliation were drawn from a variety of social and economic backgrounds: men and women; college graduates and those without degrees; people earning less than $30,000 (£19,000) and more than $75,000 per year.