Now, a group of House members led by Rep. Jason Crow (D-Colo.) is turning up the pressure. In a letter to Biden on Thursday, 10 Republican and Democratic lawmakers urged the president to quickly greenlight ATACMS.

“The war in Ukraine has become a conflict of grinding attrition. We can and must help break this stalemate. By swiftly providing the Ukrainian forces with these additional capabilities, we can significantly improve their chances of victory, restore peace to Europe, and ensure a more stable and prosperous world,” the lawmakers wrote.

Advocates of sending ATACMS say that because they are fired from High Mobility Artillery Rocket Systems already sent to Ukraine, they don’t require extensive training and or logistical chains, and could be fielded immediately.

The letter also calls on the president to approve other advanced weaponry, including U.S.-made F-16 fighter jets and additional air defense capabilities such as another Patriot missile defense system. It also urges him to expedite the transfer of U.S. M1 Abrams tanks, which are expected to arrive in Ukraine by the end of the year.

U.S. officials have become less concerned that sending more advanced weaponry might escalate the conflict, as Ukraine proved repeatedly that it would not use U.S. equipment to strike into Russian territory, Crow said in an interview.

The reluctance to send ATACMS now stems more from Pentagon fears that the U.S. has a limited number of missiles in its stockpiles, he acknowledged. But Crow’s counterargument is that it’s worth the short-term risk of sending ATACMS in order to help Ukraine win the war. The Pentagon’s long-term plan is to transition away from ATACMS to more advanced missiles in the pipeline anyway, he said.

“I think it’s an acceptable risk to transfer those out of our inventory, send them to Ukraine to help Ukraine fight this war while we transition to our next generation of long range fires,” Crow said. “My view is this is the war that the Ukrainians have to fight now with substantial national security implications for us and our allies.”

Kyiv itself is also making a new push for ATACMS. Ukrainian military officers told House Foreign Affairs Committee Chair Mike McCaul (R-Texas) in a recent teleconference that they need the weapons to prosecute their counteroffensive against Russia’s invasion force and to cut off Russian access to Crimea, he said.

“I just got briefed by Ukrainian forces and they desperately need ATACMS,” McCaul said in an interview. McCaul did not sign the letter as it was open only to members of the For Country Caucus.

Enabling Kyiv’s battlefield successes would also buoy public opinion in the U.S. in favor of more Ukraine military aid money, when Biden asks Congress in the fall for a new supplemental, McCaul noted.

“What happens in this counteroffensive is going to be key in terms of support from the American people. So that’s why I’ve been a big proponent of the administration giving them everything they need, which they have not,” McCaul said. “They still want long-range artillery to hit Crimea because they want to cut the land bridge between Russia and Crimea.”

McCaul says that pressure is intensifying, from both Democrats and Republicans.

“There are a lot of Democrats that want this administration to be doing more, because the longer they drag it out, that’s what [Vladimir] Putin wants,” McCaul said.

Republican and Democratic House members also urged Biden in a letter last month to send ATACMS to Ukraine, following their visit to Kyiv. The lawmakers — Reps. Joe Wilson (R-S.C.), Steve Cohen (D-Tenn.) and Victoria Spartz (R-Ind.) — acknowledged administration concerns that the weapons are in short supply but suggested Washington could overcome the issue by creating a pool of missiles with a coalition of international allies.

Crow, in his letter, didn’t limit the request to ATACMS, and also said that sending modern fighter jets will be critical for Ukraine over the long term. Biden recently said the U.S. would support a multinational program to train Ukrainian pilots on modern aircraft, including F-16s. But the U.S. would also need to approve the transfer of aircraft from any third-party country to Kyiv — something Washington has not explicitly said it would do.

“The ability to control skies and have air superiority over Ukrainian territory is going to be essential, and they certainly cannot do that with the old fighters that they have right now,” said Crow.

In the letter, lawmakers also said they were encouraged by Biden’s decision to send 31 Abrams tanks to Ukraine, but expressed concern that the weapons will not reach the battlefield until early fall.

“We strongly urge you to do everything in your power to accelerate this timeline, whether that be through the Defense Production Act, expended contracts, or pulling from different stocks to provide Ukrainian troops the tactical equipment needed to mount their counter-offensive,” the lawmakers wrote.

They also urged Biden to provide additional Patriot systems and munitions to Ukraine to help defend their cities and infrastructure from Russian attacks. The U.S. has so far sent one Patriot system to Ukraine, while European countries have also sent one.

One area Crow’s letter didn’t cover was cluster munitions, a contrast with McCaul and a small but growing number of lawmakers who favor sending the weapon to Ukraine.

Cluster munitions are a highly controversial class of weapons that are banned by more than 120 countries. Critics say they kill indiscriminately and endanger civilians because their bomblets can fail to explode, littering battlefields for years.

The Biden administration is said to be concerned that providing them would fracture support for Ukraine aid among allies and in Congress.

Crow, a former Army Ranger and Afghanistan veteran, said the long-term risks and costs outweigh the immediate benefit.

“I think using them are kind of undermines the moral high ground the Ukrainians have done a good job of maintaining,” Crow said, adding that “I spent formative years of my life in Afghanistan looking at kids seeing young Afghan children walking around without arms and legs decades after cluster munitions were used by the Russians in the ‘80s, and I don’t want to see that with Ukrainian children.”

But HASC’s top Democrat, Rep. Adam Smith, said in an interview last week he’s “close” to favoring cluster munitions for Ukraine. However, he said he worries about the safety issues and shares the Biden administration’s concerns that providing them would fracture support for Ukraine aid among allies and in Congress.

“What happens to the coalition, because a lot of European countries are part of international treaties that say don’t ever use cluster bombs,” Smith said. “Are they going to be OK with that? Is it going to damage the ability of the coalition to stay together? And then also how do my colleagues in the House feel about it? Could it potentially undermine overall support for Ukraine?”

Connor O’Brien and Alexander Ward contributed to this report.