Allegations that the justice department is blocking judges on political grounds comes as the attorney general has drastically tightened asylum rules

The Trump administration has been accused of illegally rejecting potential immigration judges for being too liberal, amid outrage over families being forcibly separated at the southern US border and a harsh crackdown on migrants seeking asylum.

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The allegations of politicized hiring practices come as the attorney general, Jeff Sessions, has issued a decision that will bar most victims of gangs or domestic violence from securing asylum in the US.

As chaos continues at the Mexican border over Trump’s policy of “zero tolerance” against anyone entering the US unlawfully, Session has underlined his power over a quasi-judicial immigration system that affects millions.

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The attorney general has the power to hire and fire immigration judges and make unilateral rulings such as the one he announced last Monday, which holds that those who cross the border illegally seeking sanctuary as victims of “private criminal activity”, as opposed to state-sanctioned persecution, will not generally qualify for asylum.

Critics now worry that immigration courts are vulnerable to politicization by a White House with an overt hostility towards many classes of immigrants.

Four Democratic members of Congress wrote to Sessions in April expressing concern after whistleblower allegations that the Department of Justice “may be using ideological and political considerations to improperly – and illegally – block the hiring of immigration judges and members of the Board of Immigration Appeals”.

The Trump administration has made increasing the number of immigration judges a priority in order to deal with a huge pile-up of cases. Currently about 350 judges in 60 US courts are tasked with handling a record backlog of more than 714,000 cases, according to Syracuse University’s Trac database. That is an increase of more than 171,000 cases since Donald Trump took office in January 2017.

The problem is so acute that Sessions introduced an annual quota of at least 700 completed cases per judge in order for their performance to be graded “satisfactory” – which will put pressure on them to rush through complex cases, as asylum claims often are.

Nevertheless, the Democratic lawmakers found evidence of unusual delays in the hiring process, and cited allegations that candidates were being rejected “based on their perceived political or ideological views”, which they interpreted as implying they were not conservative enough. The claims evoke a scandal from 2008, when a government report found evidence of political hiring in the Department of Justice during the George W Bush presidency, with aides appointing immigration judges and prosecutors with conservative views on topics such as abortion and same-sex marriage.

Fox News last week obtained a response on the issue from the Department of Justice which said that 14 applicants since December 2016 were no longer under consideration for reasons such as voluntary withdrawal and concerns about personal conduct raised by background checks. Assistant attorney general Stephen Boyd said, according to Fox: “The Department of Justice does not discriminate on the basis of political affiliation.”

Facebook Twitter Pinterest Undocumented immigrants leave a US federal court in shackles last week in McAllen, Texas. Photograph: John Moore/Getty Images

In response, two congressmen , Lloyd Doggett and Elijah Cummings, issued a joint statement which said that the response did not fully address their concerns, and called for an independent investigation by the office of the inspector general.

The Guardian spoke to one rejected judicial candidate, who requested anonymity. He was made a provisional offer during the Obama presidency but said his application was then subject to inexplicable delays last year. He said he was ultimately turned down with scant explanation and not given an opportunity to respond to any potential concerns about his background.

“I was never asked to explain any problems and was given to understand there was no problem with my file … The court system is run by the DoJ but it’s not supposed to be an enforcement agency, it’s supposed to be a court,” he said.

Sessions has sought to streamline the hiring process but has introduced a requirement that immigration judges have litigation experience. Biographies of new judges published by his department show many formerly worked as government prosecutors, frequently for Immigration and Customs Enforcement (Ice) – the federal agency that carries out detentions and deportations.

The president of the National Association of Immigration Judges, Ashley Tabaddor, said her group supports independence from the justice department. “What we point to is that a traditional American judicial model recognizes that for the model to have true integrity and respect the judges have to be independent,” she told the Guardian.

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Sessions’s ruling has the potential to affect tens of thousands of pending cases. “I do anticipate it to have a direct impact because in a large percentage of the individuals from Central America who are filing for asylum, their fear of return tends to be based on the fear of gang violence or domestic violence,” Tabaddor said.

Sessions argues that reform is necessary to stop what he views as a gaming of the system by migrants dubiously claiming a “credible fear” of persecution if they return to their home countries. However, 16 retired immigration judges and former members of the Board of Immigration Appeals signed a statement calling his asylum ruling “an affront to the rule of law”.

The new position opens the door for many asylum seekers to be rejected at the initial screening stage, which occurs long before a court might take up a case. Already, the majority of asylum-seekers who do reach court are turned down. Figures from Trac show that the denial rate in asylum cases is rising and success rates vary wildly by nationality, location and judge. Over a six-year period in San Francisco for example, one judge had a denial rate of 9.4% while another issued denials 97.1% of the time, according to the research.

The number of applicants without legal representation is also increasing. In another key difference between immigration courts and criminal courts, the government need not provide public defenders for people without an attorney.

“The decision underscores the importance of representation by competent attorneys now more than ever,” said Geoffrey Hoffman, immigration clinic director at the University of Houston Law Center.