A new census of Muslim congregations now under way is reviving controversies over how many Muslims are in the United States, how they are counted, and why it matters.

For minority religious groups, particularly Muslims and Jews, higher numbers can mean enhanced social and political clout in the U.S. public square.

On the campaign trail, will a politician stop by a synagogue or a mosque? When members of Congress vote on Middle East policy, which home state constituency has more influence? When the school board sets next year's vacation calendar, whose holy days are recognized?

"Numbers are a major factor in being marginalized or being recognized by decision-makers in public policy," says Ibrahim Hooper, spokesman for the Council for American-Islamic Relations, a civil rights and advocacy group and a sponsor of this second mosque census.

The first census in 2000 counted 1,209 mosques. The tally became controversial when the census announced more than 6 million Muslims were in the United States — a figure was so high it would have vaulted Islam into one of the five largest religious denominations in the country.

But lead researcher Ihsan Bagby, now associate professor of Islamic studies at the University of Kentucky-Lexington, says the 6 million figure was actually a "ballpark estimate." Bagby says he "extrapolated" the total from the 1.8 million adults counted in attendance by imams and mosque leaders. He wrapped in estimated numbers of family members who didn't attend, people in prison and people who may only go to the mosque for major festivals. No other national survey has counted anything close to Bagby's numbers.

Because the U.S. census does not ask about religion, every tally is to some degree an estimate based on institutional or individual reporting. And every way of counting and determining who should be counted have their critics. There is no agreement among or even within denominations on who is an authentic "member," says David Roozen, director of the Hartford (Conn.) Institute for Religious Research.

The institute is overseeing the mosque study as part of updating its comprehensive look at congregations to examine their fundamental role in American life.

The Catholic Church has many ways dioceses report the number of Catholics. Islam, Hinduism and Buddhism historically do not follow a traditional Judeo-Christian model of "membership" in an institution. Immigrants are unwilling or unable to participate in a telephone survey. Some faiths count infants, others count only adults. Some apply a theological screen to who is a member, insisting that merely attending worship is not sufficient.

And no one agrees on whether people born and reared in a denomination who now say they have no religion should be counted with their parents based on "common heritage and historic experience and social, political ties," says Ira Sheskin, geography professor at University of Miami and director of the Jewish Demography Project.

"What people do and how they think of themselves, and how we want to classify their institutional affiliations are very, very different," says Gary Tobin, president of the Institute for Jewish and Community Research in San Francisco. "If you say you are Episcopalian, but you attend a Baptist church on Sundays with your wife who is Baptist, who counts you? If neither one of you attends church anywhere, what are you then?"

Bagby adds: "Everyone makes judgment calls about who is counted."

The mosque census is being conducted with telephone and Internet questionnaires completed by imams, staff and other key people at mosques.

"We are looking at the numbers at prayer, ethnicity, the role of the staff, women's participation and level of involvement in American civic life," Bagby says.