While still outnumbering the next biggest group - Catholics - roughly two to one, Protestant denominations have been losing members to people who say they have no religion, the report from the University of Chicago said.
While Protestant membership stood at 63% of the population in 1993, it fell to 52% in 2002 and will drop below half in the next year or two, if that hasn't happened already, it added.
The information came from a survey that has been tracking societal trends for 32 years.
Tom Smith, general director of the National Opinion Research Center, said the Protestant decline is another example of how the United States is on it way to being a nation of minorities.
"Many scholars have noted that the numbers of people who say they have 'no religion' is increasing, but they haven't noted what faith group these people have been leaving.
"It is clear that many of these people are former Protestants," he said.
It is also possible a small number of the people who formerly identified themselves as Protestant have now decided to identify themselves simply as "Christian" - in which case they would be in the "other" category on the survey, Smith said.
President Bush is a practising
The survey found those who said they were Catholic in 2002 remained fairly steady at about 25% of the population.
People who said they belonged to other religions, including Eastern faiths and Islam, Orthodox Christians, interdenominational Christians and native-American faiths, increased from 3% to 7% between 1993 and 2002.
Meanwhile, the number of people who said they were Jewish remained stable at slightly under 2%.
Those who said they identified with no religion totalled nearly 14% in 2002 compared to 9% in 1993.
Protestants are in decline, the survey found, because younger adherents are dropping out. Immigration will probably further dilute the Protestant numbers but will keep Catholic rates stable, Smith said.
"Many scholars have noted that the numbers of people who say they have 'no religion' is increasing, but they haven't noted what faith group these people have been leaving. It is clear that many of these people are former Protestants"
National Opinion Research Center
The study defined Protestants as those belonging to all post-Reformation Christian churches such as Baptist, Methodist and Episcopalian, including Mormons and New Age Spirituality adherents.
The survey included 2765 people who were 18 and older and there was a margin of error of plus or minus 2% for the 2002 statistics.