Protestant affiliation has dropped sharply in the US over the past 15 years, with only 36 per cent of Americans identifying themselves as Protestant last year compared to 50 per cent in 2003, according to a new analysis for ABC News.

The decline among what remains the nation's prevalent religious group includes an eight-point drop in the number of evangelical white Protestants, the study of ABC News and Washington Post polls by Langer Research Associates shows.

Meanwhile, the share of Christians overall has declined from 83 per cent of the adult population in 2003, to 72 per cent on average last year. Over the same period, the number of Americans who say they have no religion has nearly doubled, to 21 per cent.

Catholic self-identification has held steady during this time, at 22 per cent. The share of adults who identify with another form of Christianity – including Jehovah Witnesses, Mormons and Greek or Russian Orthodox – has risen modestly, from 11 to 14 percent.

This analysis is based on a large dataset – 174,485 random-sample telephone interviews in ABC News and ABC News/Washington Post polls conducted from 2003 to 2017.

Among all Protestants, 56 per cent currently say they are evangelical or born-again; that has also held steady since 2003, with virtually equal declines in the number who say they are either evangelical or non-evangelical Protestants, down seven and six points respectively.

Evangelical white Protestants – 80 per cent of whom supported Donald Trump's election as US president – have seen their share of the total adult population go down from 21 per cent in 2003, to 13 per cent last year. Meanwhile, non-evangelical white Protestants have gone from 17 to 11 per cent.

According to the study, most Protestants do not identify themselves as Protestant but as a member of a particular Protestant denomination – Baptist, Methodist, Lutheran, Pentecostalist, Presbyterian, Episcopalian and others. 

The analysis also pointed out that stability in the share of the population that is Catholic is in part at least down to the fact that half of Hispanics identify themselves as Catholic. However, even among non-Hispanic whites, the share of Catholics has held nearly steady – 22 per cent in 2003 and 20 per cent now.

The number of adults expressing no religious affiliation has risen from 12 per cent in 2003 to 21 per cent of all adults in 2017; that includes three per cent who say they are atheists, three per cent agnostic and 15 per cent who say they have no religion. The proportions were similar 15 years ago, according to the data.

As is to be expected, evangelical white Protestants are a core Republican group; 48 per cent identify themselves as Republicans, 31 per cent as political independents and just 14 per cent as Democrats. Similarly, 53 per cent of Mormons are Republicans, 34 per cent independents and nine per cent Democrats.