The Trump administration and Congress are on yet another collision course over U.S. sanctions targeting Russia, but this time American allies are caught in the middle.

Defense Secretary James Mattis is warning that India and other major defense partners could face tough U.S. sanctions for their impending purchases of new military equipment from sanctioned Russian companies, and has asked Congress for the authority to waive those sanctions. But top lawmakers have signalled that they aren’t willing to give the administration broad power to weaken a tough U.S. sanctions regime enacted last year and aimed at punishing Russia’s defense and intelligence sectors over Moscow’s interference in the 2016 U.S. election and its incursions into eastern Europe.

Last week, Mattis asked Congress to give the administration a national-security waiver that would allow some U.S. allies to purchase military equipment from Russian companies without facing U.S. sanctions. The State Department designated those Russian entities for sanctions last year under a bipartisan law that mandates strict financial punishments against the country.

Congressional leaders are discussing Mattis’ request, two sources familiar with those deliberations told The Daily Beast, but they aren’t likely to give the administration what it wants.

Democrats and Republicans are united in their skepticism over the administration’s latest attempt to take back some of the authority it ceded to Congress last summer when Trump reluctantly signed the Countering America’s Adversaries Through Sanctions Act (CAATSA) into law. The escalating row represents a major stress test for Congress’ role as a check on the Trump administration’s Russia policies.

Mattis’ appeal to Congress comes as India is preparing to buy a missile defense system, known as the S-400, from Russia. Under CAATSA, India would immediately face U.S. sanctions for the purchase. Mattis argued that this would harm U.S. interests and would be counter-productive to the goal of CAATSA—to limit foreign investments in Russian defense and intelligence firms—because sanctioning a strategic partner would push that country to buy even more Russian equipment. It could also prevent those nations from buying American equipment in the future, he warned.

“We only need to look at India, Vietnam and some others to recognize that eventually we’re going to paralyze ourselves,” Mattis said last week at a Senate Armed Services Committee hearing, noting that India currently operates so-called “legacy” Russian systems that need to be revamped with the same type of equipment.

A waiver like the one Mattis is requesting would give the administration back some of the power it previously had over sanctions-related policymaking. More specifically, it would allow the State Department and the Pentagon to waive U.S. sanctions against countries such as India, Indonesia and Vietnam which are preparing to upgrade their existing Russian-made military infrastructure.

But that will be a tough sell for congressional Republicans, who argue that the sanctions regime is working as written, as well as for Democrats, who are wary to grant the Trump administration more power over Russia-related matters.

Mattis’ request, which would apply to more U.S. allies than just India, would require brand new legislation from Congress because granting the administration that authority is prohibited under the current law. The administration has not yet asked the relevant congressional committees for a specific waiver for India’s purchase, either, five congressional sources with direct knowledge of the matter told The Daily Beast.

A senior GOP congressional aide said the administration “will need to make a detailed, public case for relief” in order for Congress to seriously consider a waiver. A broader request would be even tougher. Many on Capitol Hill have made the case that the law is working as it was intended by preventing the administration from taking unilateral action without first consulting top lawmakers.

“Congress could be convinced, but you’d have to have a very detailed and compelling case from the administration and from Secretary Mattis on why this would be needed,” the Republican aide said, noting that lawmakers would approach the matter with deep skepticism.

Democrats on Capitol Hill pointed out that the CAATSA law was crafted such that it gives the Trump administration the tools to resolve the India situation diplomatically. The law allows the waiving of sanctions if countries currently doing business with Russian entities can prove that they are winding down their financial relationships with those companies. Democrats say it’s up to the administration to pressure India’s government to not buy Russian equipment, instead of asking Congress to revise a law that the president already signed.

“No one wants to sanction our closest allies. The point is that we gave the administration tools under CAATSA to use diplomatic leverage to get these countries to change their behavior,” a Senate Democratic aide said. “The administration just isn’t doing the hard work of diplomacy.”

The law stipulates that in order to exercise waiver authority without congressional involvement, the administration would have to certify that Russia has ceased its cyberattacks. And because that’s far from the case, Mattis appears to be searching for a work-around—and lawmakers say they saw it coming.

“CAATSA was one of the first times that Congress, with a Republican majority, decided to have mandatory sanctions not subject to waivers. Why did they agree to do that with a Republican Congress? Because they feared that the president would not, in fact, enforce sanctions,” Sen. Bob Menendez (D-.N.J.), the top Democrat on the Senate Foreign Relations Committee, told The Daily Beast.

Lawmakers and the administration have been at odds over the waiver provisions in CAATSA ever since the law was first being written last spring. Negotiations on the waivers were so fraught at one point that some wondered whether the effort would even get off the ground. The Daily Beast first reported on efforts by top Trump administration officials to weaken key provisions of the legislation which were intended to give Congress the authority to review any attempts by the executive branch to alter the sanctions regime.

But Mattis wants some of that authority back. At his Senate hearing last week, the defense secretary asked Congress to pass a national security waiver that would give Secretary of State Mike Pompeo the power to unilaterally nix certain sanctions.

“So if [Pompeo] has the waiver authority and I can go to him and show it’s in our best interest, then we get an internal management of this process,” Mattis added. “But it keeps us from being boxed in by the Russians.”

A State Department spokesperson declined to comment when asked if it had sent any waiver requests to Congress, but said the position of the department remains that “significant transactions with listed Russian entities will result in sanctions.” The spokesperson also declined to comment on India’s expected purchase of the S-400 system from Russia. The Pentagon did not respond to multiple requests for comment.

The legislation passed with overwhelming majorities in both the House and Senate last summer, and Trump reluctantly signed the bill after disparaging it as congressional overreach. Since then, the administration has missed key deadlines to issue guidance for the sanctions, and more recently the State Department declined to implement those financial punishments against Russia at the January 30 deadline. It later implemented some of those measures after the U.S. and the United Kingdom deemed Russia responsible for the nerve-agent attack on a former Russian spy on British soil.

The administration’s overall posture toward the CAATSA law has drawn swift condemnation from lawmakers on both sides of the aisle, and more recently spurred the drafting of a House resolution aimed at admonishing the administration for not implementing the congressionally mandated sanctions.