WASHINGTON, August 11, 2010— -- Eight percent of all babies born in the U.S. in 2008 belonged to illegal immigrant parents, according to a groundbreaking analysis of U.S. Census Bureau data by the Pew Hispanic Center. Under the 14th amendment to the Constitution, each child obtained U.S. citizenship at birth while one or both of the parents remained undocumented.
The study sheds new light on a group of Americans at the center of a hot political debate in recent weeks. Some Republican lawmakers have proposed revising birthright citizenship to bar U.S.-born children of illegal immigrants from obtaining legal status.
Pew estimates 340,000 of the 4.3 million newborns in U.S. hospitals in 2008 belonged to illegal immigrant parents. In total, 4 million U.S.-born, citizen children of illegal immigrants currently live in the country, according to the study.
The study is the most comprehensive, non-partisan research to date on children of illegal immigrants living in the U.S. and adds important context, and frames the ongoing debate. Previously there have been few reliable estimates of annual U.S. births to illegal immigrants.
Critics of birthright citizenship have expressed concern over the burgeoning size of America's illegal immigrant population, estimated at 10.8 million and whose offspring in the U.S. would be able to sponsor their parents and relatives for legal residency. The children are sometimes referred to as anchor babies.
"Birthright citizenship I think is a mistake," said Republican Sen. Lindsey Graham of South Carolina. "We should change our Constitution and say if you come here illegally and you have a child, that child's automatically not a citizen."
Senate Minority Leader Mitch McConnell and other leading Republicans, including Arizona senators Jon Kyl and John McCain, have indicated an openness to exploring the 14th Amendment issue raised by Sen. Graham. But some harbor deep reservations about changing the Constitution.
"It's a rather unseemly business and I think we ought to have some hearings about it," McConnell said of the practice of illegal immigrant mothers giving birth in the U.S.
"Congressional hearings are always warranted when members of Congress raise the issue of amending our Constitution," said McCain in a statement. "I believe that the Constitution is a strong, complete and carefully crafted document that has successfully governed our nation for centuries and any proposal to amend the Constitution should receive extensive and thoughtful consideration."
Birthright Citizenship Debate: Election Year Politics?
But some lawmakers are calling the push to revise the 14th Amendment nothing but a political stunt.
"I think it's good to take a look at all of our constitutional amendments. But I'll tell you something: if you think it's a coincidence that this sudden discussion begins three months before an election, you'd be very, very mistaken," said Vermont Independent Sen. Bernie Sanders on ABC's "Top Line".
Pennsylvannia Democratic Sen. Arlen Specter, whose parents were immigrants to the U.S., has called U.S. citizenship by birth a fundamental right.
"The political pandering on the immigration issue has reached the hysterical level," Specter told ABC News. "To try to direct the effort at the children born in this country is just preposterous... How can newborn children protect themselves if politicians want to gain political gain... I would be shocked if this idea would gain political traction, but I'm being shocked on a daily basis by the United States Senate."
The 14th Amendment to the Constitution, which was enacted after the Civil War to grant citizenship to descendants of slaves, reads: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."
"The drafter of the 14th Amendment provision on citizenship did make a statement that it would not include foreigners or aliens," said George Washington University constitutional law professor Jonathan Turley. "However, other senators made it clear that they believed that this provision guaranteed birthright citizenship."
The courts have repeatedly ruled that people who are born in the U.S. are American citizens and if Congress passed a law changing that, it would likely be repealed, experts say.
The Supreme Court has only addressed the issue once, clarifying in 1898 that citizenship does apply to U.S.-born children of legal immigrants who have yet to become citizens.
"The legislative history may be a little mixed, but the language of the amendment seems to speak clearly in favor of birthright citizenship, regardless of what the intent may have been," Turley said.
The United States is one of the few remaining countries to grant citizenship to all children born on its soil. The United Kingdom, Ireland, India and Australia, among others, have since revised their birthright laws, no longer allowing every child born on their soil to get citizenship.
More stories from ABC News' special series "Out of the Shadows: Illegal Immigration in America"