The State Department is denying passports to a large number of people with official US birth certificates that state they were born in South Texas in the past 70 years.

Texas congressmen said the government's policy, reported by The Washington Post on Wednesday, is part of a systemic anti-Hispanic bias that has guided the administration's immigration policy and suggested they would propose legislation to address the policy.

"This represents an unacceptable targeting of people based on their ethnic heritage. It violates the Constitution. It should be investigated by Congress in both chambers, and we should take action to stop it as soon as possible through legislation if necessary," said Rep Joaquin Castro, D-Texas.

But Republican leadership in the House and Senate is unlikely to bow to Democratic demands on any proposed legislation on the issue.

The government alleges that decades ago, midwives and some doctors in South Texas provided fraudulent birth certificates to babies who were actually born in Mexico. In some cases, in the 1990s, several of those birth attendants pleaded guilty to fraud.

But the same midwives and doctors accused of fraud also delivered tens of thousands of babies in Texas and decades later, it is nearly impossible to distinguish between the relatively small number of fraudulent birth certificates and the swell of legal ones. That has left US military veterans, Border Patrol agents and police officers on a frenzied quest to prove that they were born in the United States.

The policy of targeting people with birth certificates signed by midwives existed under both the George W Bush and Barack Obama administrations, but in 2009 the US government reached a settlement with the American Civil Liberties Union that appeared to resolve the issue. Lawyers along the border say the number of passport denials declined during the last year of the Obama administration and it became easier for those questioned to substantiate their birth in the United States without going to court.

The State Department initially declined a Washington Post request for statistics on passport denials. "To ensure the integrity of the passport, the Department does not publish fraud trend statistics," said a State Department statement.

But after the publication of the Post report on August 29, the State Department challenged the findings and issued data on August 31 suggesting passport denials were at the lowest level in years.

A document labelled "Domestic issuance/denials along the southern border involving potentially fraudulent birth documents" said that in 2017, 971 people, or 28 per cent of those in the category, were denied passports, a smaller percentage than in any of the four previous years.

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"The State Department's domestic passport denials are at the lowest rate in six years for midwife cases," said State Department spokesperson Heather Nauert in a statement after the story was published.

But those numbers appear to leave out key data. The State Department declined repeated requests from The Post for additional information.

In the majority of cases reviewed by The Post, passport applicants delivered by midwives in South Texas receive repeated requests for additional documentation, but never receive formal denials from the State Department.

The State Department did not respond to requests for statistics on these cases.

The State Department also would not provide details on who falls into the category of "potentially fraudulent birth documents" along the southern border, which it included on its data sheet.

In some cases reviewed by The Post, US military veterans and other Americans with birth certificates from cities hundreds of miles from the border were also denied. It is unclear whether such cases would be included in the newly published statistics.

Under the Trump administration, attorneys say, the number of denials has risen and the government has made it difficult for people to receive their passports without suing the State Department - an expensive undertaking for Americans whose citizenship is being questioned. Lawyers also say that some people who have been denied passports have landed in deportation proceedings.

Senator Jeff Merkley, D-Ore, tweeted: "@realDonaldTrump's war on Latino Americans gets even uglier. What happened to 'unalienable rights?' "

Republican Senator Ted Cruz of Texas echoed the State Department's concerns about fraud.

"It is important to ensure that the rights of all US citizens are respected and protected, while also preventing fraud by people who are not in our country legally," Mr Cruz said. "If US citizens are being denied their passports, that needs to stop. But if someone is not a US citizen, then their passport request should be denied."

Senator John Cornyn, R-Texas, did not respond to a request for comment.

The State Department declined to provide information about the number of people denied passports because they were delivered by midwives along the border. It said its policy has not changed.

"The US-Mexico border region happens to be an area of the country where there has been a significant incidence of citizenship fraud," it said in a statement to The Post. That explanation did not allay concerns among congressional Democrats.

"The idea that because some few people falsified documents has created a presumption that all documents issued in a region are to be questioned is like saying someone is guilty until proven innocent," said Rep Vicente Gonzalez, D-Texas. "I plan to work with my colleagues on both sides of the aisle to utilise our powers of congressional oversight and end these unwarranted actions that have gone on for far too long."

Other members of Congress said they would like to hold hearings on the topic.

"We're challenging citizens of the US who have been citizens of this country for years and have gotten a passport in years past," said Rep Sheila Jackson Lee, D-Texas. "There's no basis saying they're violating security. I'm going to ask for hearings to be held on the Judiciary Committee and immigration subcommittee on homeland security.

"I am really concerned about this. It is another tell-tale sign of the administration trying to block legal immigration and citizens just because they happen to be Hispanic or Muslim, and that's tragic and that's not America," she said.