Each year, an estimated 400,000 babies are born in the United States to mothers who are illegal immigrants. More than 25 percent of those babies are born in California.

Although Congress has never passed a law saying so, no president has ever ordered it, and no court has ever ruled on the issue, each of these babies automatically becomes a U.S. citizen when it takes its first breath.

That could change if legislation that Rep. Dan Lungren, R-Gold River, joined in sponsoring in April becomes law.

Lungren, who served as California's attorney general from 1990 to 1998 and worked on the last successful push for an immigration overhaul when he represented Long Beach in the Congress in the 1980s, said he is trying to stimulate a debate on what he believes is a key factor drawing immigrants illegally across the U.S.-Mexico border.

Critics have another word for it.

"Xenophobia," said Crystal Williams, deputy director of the American Immigration Lawyers Association. "They are afraid of having more Hispanic citizens."

Contributing to the evolution of U.S. policy is the 14th Amendment to the Constitution. Ratified in 1868, after the Civil War, it was intended to ensure that children of freed slaves were extended U.S. citizenship. The amendment states simply that anyone born in the United States and "subject to the jurisdiction thereof," is a citizen.

U.S. policy has come to recognize "birthright citizenship" in just about every vital transaction.

Need a passport? The State Department requires a birth certificate.

Need a driver's license? Same thing. A U.S. birth certificate equals citizenship.

Steven Camarota, director of research for the Center for Immigration Studies, called it "citizenship by bureaucratic default" because the policy is not the deliberate result of laws being passed, executive orders issued by the White House or court rulings.

The only Supreme Court case dealing with the 14th Amendment on the issue was in 1896, and it was limited to legal permanent aliens, Camarota said.

Many organizations concerned about the growing number of illegal immigrants crossing the U.S.-Mexico border believe that there are two ways to curb the problem. The first is to crack down on employers who hire them; the second is to repeal the 14th Amendment.

"To deal with this tidal wave of human beings coming across the border, repealing the 14th Amendment would be an effective tool," said Bob Dane, spokesman for the Federation for American Immigration Reform.

"It would not harm those coming here legally," he said. "The only beneficiaries of the 14th Amendment appear to be illegals."

In April, Lungren joined Republican Rep. Nathan Deal of Georgia and fellow California Republican Brian Bilbray of Carlsbad, in introducing legislation that stops well short of repealing the 14th Amendment. Instead, it calls for defining what "subject to the jurisdiction thereof" means.

The legislation declares that the clause would apply to any person born to a parent who is a citizen, a legal alien or an alien serving in the military.

Lungren said he thinks that provision would pass constitutional muster, noting that there already exists an exclusion to the 14th Amendment for children of foreign diplomats and representatives born in the United States.

But absent that single exclusion, Lungren said the way the 14th Amendment has been commonly viewed "is a tremendous incentive for someone who wants to give their children a better opportunity in life."

"If you have a chance to raise your child in the United States rather than Mexico for the foreseeable future, it seems to me I would rather raise them here in the United States," he said. "Some would say that is a natural instinct."

While virtually everyone agrees such a law would hit the courts soon after it was signed into law, there is great uncertainty over how the courts would rule.

This is not the first time such legislation has been introduced. Bills to either repeal or define the 14th Amendment have been introduced in every Congress for the last decade. And each year the support for them increases.

Last year, Deal's legislation drew 87 supporters. This year, the number of co-sponsors grew from 76 to a record 90 last week, after the House returned from its August recess.

So far, the bill remains a statement rather than an opportunity. No Democrat has signed onto the bill. While 11 of California's 19 Republicans have signed on, support is weakest in the agriculturally rich Central Valley areas where Lungren and Rep. Wally Herger of Marysville are the only Republican co-sponsors so far.

"I am not trying to be controversial," said Lungren. "I've introduced it for the purposes of beginning a debate. If we get off on the wrong foot, with the extremes on both sides, we will not be successful in getting it considered. But if we can get people seriously considering this, I think we stand a chance of perhaps passing this."

That's not likely in a Congress controlled by Democrats. It didn't happen in 2005 when Republicans held the gavel, either. In December 2005, House Republican leaders yanked Deal's legislation from consideration as part of a bigger borders-protection bill.

Camarota, of the Center for Immigration Studies, is not certain that passage of the legislation would do much anyway, because it deals with such a small slice of the overwhelmingly complex immigration problem.

"This is the kind of problem you run into when you neglect to enforce immigration laws," he said. "But if you're not going to do that, I don't see how denying birthright citizenship helps."

But Lungren said enforcing bans on employment of illegal aliens goes together with the legislation to end the two largest "magnets" drawing workers across the border illegally -- jobs and citizenship.

"I'm trying to look at reasonable means of controlling our borders and still adhere to this idea of this being a nation of immigrants," he said.