Raleigh, N.C. ? Nine justices, no Protestants.

If the Senate confirms Solicitor General Elena Kagan as the next Supreme Court justice, the result will be an anomaly in a country that has been dominated by Protestants since the Pilgrims.

Kagan, nominated Monday by President Barack Obama, would be the third Jewish justice, and would join six Roman Catholics on the court, meaning none of the justices would be rooted in the Protestant Reformation traditions that shaped the country from its earliest stages.

That could show Americans are far more accepting of religious diversity than they were a generation ago, when John F. Kennedy’s presidential campaign was dogged by suspicions about his Catholic faith. But it also may be a sign of the approaching time when Protestants lose their majority status.

“I don’t think this means Protestant America is over, but I do think it means the old way of thinking about Protestant America is over,” said Stephen Prothero, a professor of religion at Boston University and author of “God Is Not One.”

The possibility of a Supreme Court without Protestants is just a dramatic illustration of what observers of religion have known for years, Prothero said: that in a country where President John Adams could once say Roman Catholics were “as rare as a comet or an earthquake,” Protestants have seen their share of the population dramatically shrink.

“The population is probably just over 50 percent Protestant,” said Mark Chaves, a professor at Duke University who directs the National Congregations Study. “It’s heading down, though, and any minute now it will probably drop below 50 percent.”

Membership in “mainline” Protestant churches has been falling for decades, sapping the ranks of Episcopalians, Congregationalists, Presbyterians, Methodists, Lutherans and others who for years defined American mainstream religion.

In recent years, even Evangelical Protestant churches like the Southern Baptists, which grew while mainline churches faltered, have seen membership either stall or decline.