WASHINGTON — The Justice Department lent its support on Thursday to students who are suing Harvard University over affirmative action policies that they claim discriminate against Asian-American applicants, in a case that could have far-reaching consequences for the use of affirmative action in college admissions.

In a so-called statement of interest, the department supported the claims of the plaintiffs, a group of Asian-Americans rejected by Harvard. They contend that Harvard has systematically discriminated against them by artificially capping the number of qualified Asian-Americans from attending the school in order to advance less qualified students of other races.

“Harvard has failed to carry its demanding burden to show that its use of race does not inflict unlawful racial discrimination on Asian-Americans,” the Justice Department said in its filing.

The filing said that Harvard “uses a vague ‘personal rating’ that harms Asian-American applicants’ chances for admission and may be infected with racial bias; engages in unlawful racial balancing; and has never seriously considered race-neutral alternatives in its more than 45 years of using race to make admissions decisions.”

In recent years, the Justice Department has increasingly used such statements of interest to intervene in civil rights cases. Before 2006, such statements appeared only seven times in civil rights-oriented disputes, according to a recent paper by law school student Victor Zapana. Between 2006 and 2011, they were drafted in at least 242, almost all by the Obama administration on issues such as videotaping police brutality, ensuring that blind people and their service dogs have access to Uber or preventing the police from arresting homeless people who cannot find beds in shelters.

But the Trump administration is now turning the same tool against affirmative action in college admissions, a major — and highly contentious — legacy of the civil rights era. In the past few years, affirmative action has faced a fresh barrage of scrutiny from Asian-Americans who argue that they are being held to a higher standard, losing out on coveted slots at places like Harvard as African-Americans, Latinos and other groups get a boost. Universities that factor race into admissions have also seen a powerful new opponent of the policies emerge in the Trump administration, which argued in its filing on Thursday that the court should deny Harvard’s request to dismiss the case before trial.

The government said that Supreme Court rulings require that universities considering race in admissions meet several standards. They must define their diversity-related goals and show that they cannot meet those goals without using race as a factor in admissions decisions.

The department argued that Harvard does not adequately explain how race factors into its admissions decisions, leaving open the possibility that the university is going beyond what the law allows.

“Harvard has failed to show that it does not unlawfully discriminate against Asian-Americans,” the Justice Department said in a statement Thursday.

Harvard responded, saying it was “deeply disappointed” but not surprised “given the highly irregular investigation the D.O.J. has engaged in thus far.”

“Harvard does not discriminate against applicants from any group, and will continue to vigorously defend the legal right of every college and university to consider race as one factor among many in college admissions, which the Supreme Court has consistently upheld for more than 40 years. Colleges and universities must have the freedom and flexibility to create the diverse communities that are vital to the learning experience of every student,” the university said in a statement.

The Harvard case, which was brought by an anti-affirmative-action group called Students for Fair Admissions, is seen as a test of whether a decades-long effort by conservative politicians and advocates to roll back affirmative action policies will ultimately succeed.

That push has broad support from President Trump. The Department of Education and Justice Department said in July that the administration was abandoning Obama-era policies that asked universities to consider race as a factor in diversifying their campuses and would favor race-blind admissions instead.

Officials from both departments said that the Obama administration had used guidelines to circumvent Congress and the courts to create affirmative action policies that went beyond existing law. The Trump administration contends that its stance on affirmative action adheres to the letter of the law and the opinions of the courts.

“The Supreme Court has determined what affirmative action policies are constitutional, and the court’s written decisions are the best guide for navigating this complex issue,” Education Secretary Betsy DeVos wrote in a statement when the guidance rollback was announced.

Civil rights leaders and others argue that this stance effectively undermines decades of policy progress that created opportunity for minorities, and that it has thrown race-conscious admissions into jeopardy.

The department typically files statements of interest in cases that it feels directly impact the federal government’s interests.

“As a recipient of taxpayer dollars, Harvard has a responsibility to conduct its admissions policy without racial discrimination by using meaningful admissions criteria that met lawful requirements,” Attorney General Jeff Sessions said in a statement.

In briefs filed in support of Harvard at the end of July, students and alumni said that they “condemn” the plaintiffs’ “attempt to manufacture conflict between racial and ethnic groups in order to revive an unrelenting agenda to dismantle efforts to create a racially diverse and inclusive student body through college admissions.”

“It’s alarming that Trump is aligning himself with anti-civil rights activist Edward Blum in this subversive attempt to say that civil rights protections cause discrimination,” said Jeannie Park, the head of the Harvard Asian-American Alumni Alliance, referring to the founder of Students for Fair Admissions. “Asian-Americans have long benefited from policies to increase equal opportunity and still do. Our fear is that Harvard’s admissions system is just the latest target in a larger fight to roll back protections for people of color in all fields, including government and business.”

At the heart of the case, is whether Harvard’s admissions staff hold Asian-Americans to higher standards than applicants of other racial or ethnic groups, and whether they use subjective measures, like personal scores, to cap the number of Asian students accepted to the school.

“It turns out that the suspicions of Asian-American alumni, students and applicants were right all along,” Students for Fair Admissions said in a court filing. “Harvard today engages in the same kind of discrimination and stereotyping that it used to justify quotas on Jewish applicants in the 1920s and 1930s.”

Harvard, which admitted less than 5 percent of its applicants this year, said that its own analysis did not find discrimination. The university has also noted that Mr. Blum has lost previous challenges to affirmative action policies.

A trial in the case has been scheduled for October.

If it winds its way to the Supreme Court, it could be heard by Judge Brett M. Kavanaugh, Mr. Trump’s nominee for the vacant seat once held by Justice Anthony M. Kennedy. The case could have far-reaching implications for the nation’s colleges and universities that consider race in their admissions processes.

The Justice Department is pursuing its own investigation into Harvard’s admissions policies based on a complaint it received.

A handful of states already ban public universities from relying on affirmative action, pushing several toward a model that takes socioeconomic factors into account while picking and choosing new students instead of race. Public universities in California and Washington, for example, have tried to engineer class-based diversity in their student bodies, believing that giving a lift to lower-income students will end up bringing more minority students in as well. The University of Texas, meanwhile, accepts a preset percentage of the top-ranking students at every high school in the state.

But these methods have not produced classes with an ethnic makeup that mirrors that of the states where they have been used, and many selective private universities continue to admit students partly on the basis of race — though, until Harvard was forced to detail its internal admissions policies recently, few could say how elite universities actually weighed applicants’ race.

The Obama administration used statements of interest in novel ways to further its civil rights agenda, sometimes by making the unusual move of intervening in local cases.

In 2013, the Justice Department intervened in a local case in Burlington, Wash., where poor people contended that their public lawyers were too overworked to adequately do their jobs. The judge subsequently ordered the city to hire a new public defense supervisor and placed its legal aid program under court supervision for three years.

The Obama administration also intervened in local legal disputes over legal aid issues in New York, juvenile prisoners in California and transgender students in Michigan.

The Trump administration has filed statements of interest on other charged topics this year. In February it supported the plaintiffs who are suing pharmaceutical companies and drug distributors in a sprawling, prescription opioids lawsuit in Ohio. This month is supported the merits of a suit that says Facebook abets discriminatory housing practices, in violation of the Fair Housing Act.

Correction: 

An earlier version of this article misstated when the Justice Department released a statement of interest supporting students suing Harvard in an affirmative action case. The filing was made Thursday, not Friday.