It's become routine since Russia invaded Ukraine: President Joe Biden and Ukrainian President Volodymyr Zelenskyy speak by phone whenever the U.S. announces a new package of military assistance for Kyiv.
But a phone call between the two leaders in June played out differently from previous ones, according to four people familiar with the call. Biden had barely finished telling Zelenskyy he'd just greenlighted another $1 billion in U.S. military assistance for Ukraine when Zelenskyy started listing all the additional help he needed and wasn't getting. Biden lost his temper, the people familiar with the call said. The American people were being quite generous, and his administration and the U.S. military were working hard to help Ukraine, he said, raising his voice, and Zelenskyy could show a little more gratitude.
Administration officials said Biden and Zelenskyy's relationship has only improved since the June phone call, after which Zelenskyy made a statement praising the U.S. for its generous assistance. But the clash reflects Biden's early awareness that both congressional and public support for sending billions of dollars to Ukraine could begin to fade. That moment has arrived just as the president prepares to ask Congress to greenlight even money for Ukraine.
Biden now faces resistance from some Republicans and Democrats that wasn't present when Congress approved previous Ukraine funds. The White House has discussed asking Congress for billions of dollars during the lame duck legislative session after the midterm elections.
The White House hasn't specified an amount publicly. Lawmakers and Ukraine lobbyists hope for $40 billion to $60 billion, and some officials familiar with the discussions expect the number to be roughly $50 billion.
A source familiar with the conversation said that Biden was direct with Zelenskyy about handling the issues in the appropriate military channels but that the exchange wasn't heated or angry.
A spokesperson for the National Security Council declined to comment on the story.
A spokesperson for Zelenskyy didn't respond to a request for comment.
Top U.S. officials warn there are no signs the war is ending any time soon.
Before the June 15 phone call, the president's frustrations with Zelenskyy had been building for weeks, three people familiar with the call said. Biden and some of his top aides felt that the administration was doing as much as it could as quickly as it could but that Zelenskyy continued to focus publicly on only what wasn't being done.
From Zelenskyy's perspective - as well as that of some Eastern European governments and U.S. lawmakers from both parties - there has been repeated frustration that the Biden White House moves too slowly on weapons requests, initially hesitating to approve certain capabilities Ukraine requested most urgently, only to relent weeks or months later under pressure, according to two sources familiar with the Ukraine government's view, congressional aides and two European officials.
After the pushback Zelenskyy got in their June phone call, his team decided to try to defuse tensions, concluding it wasn't productive to have friction with the U.S. president, according to two sources familiar with the Ukraine government's view, congressional aides and two European officials.
Zelenskyy responded publicly that day by thanking Biden for the promised assistance.
"I had an important conversation with U.S. President Biden today," he said in videotaped remarks. "I am grateful for this support. It is especially important for our defense in Donbas."
In his statement after the call, Biden said he had informed Zelenskyy of the $1 billion in aid and vowed the U.S. "will not waver in our commitment to the Ukrainian people as they fight for their freedom."
The effort to get Ukraine weapons and equipment has intensified in recent weeks as Ukraine tries to make significant gains before harsh winter temperatures set in.
The Ukrainian military is focused on driving thousands of Russian troops away from Kherson, trying to encircle them and retake the southern city from Russian control. The battle for Kherson could be one of the most consequential battles in Ukraine since the invasion. If Ukraine is able to retake the area, it could be a major morale booster for Zelenskyy's forces and a serious blow to Russian troop confidence. But if Russia holds on, it could maintain its grip on the south, including the Zaporizhzhia nuclear power plant, through the winter months. "This could be a turning point," a defense official said.
Concerns about fading support for Ukraine are also driving the current offensives, according to a defense official and a former official, as Ukraine tries to show momentum on the battlefield to encourage the flow of more weapons.
On Oct. 12, Defense Secretary Lloyd Austin convened a meeting of the Ukraine Contact Group in Brussels, a periodic gathering of allies, to discuss how to get more weapons and equipment into Ukrainian military hands. While past meetings have yielded assistance from ammunition to missile launchers, this month's meeting took on new urgency, according to three defense officials familiar with the discussions.
"Everyone was stepping up," said an official in the meeting. Countries were scouring their stockpiles and warehouses to find anything that could help the Ukrainian military, the official said. "There was an urgency to get them air defenses and anything we could before winter and so they can be successful in this current offensive."
The meeting was so successful that Austin was giddy as he walked out, two defense officials said.
Ukraine still needs more air defense systems to defend against Russian military aircraft, missiles and drones, and the U.S. continues to discuss providing longer-range missile systems like the ATACMS and even some advanced fighter aircraft in the future.
The proportion of Americans who are extremely or very concerned about Ukraine's losing the war has dropped by 17 percentage points since May, from 55% to 38%, according to a Pew Research Center survey conducted last month. And the proportion of Americans who say they're not too concerned or not at all concerned about Russia's winning was up from 16% to 26%, according to the survey.
The potential change in political will in the U.S. for continuing to send aid to Ukraine could upend how both the White House and Zelenskyy have approached the issue so far.
Since Russia invaded Ukraine in February, the Biden administration has been criticized for moving too cautiously. Now the president faces potential pushback from some Republican lawmakers and progressive Democrats that he's providing too much aid.
The shifting dynamics on Capitol Hill also could force Zelenskyy's team to rethink how it engages with Washington, as it has often tried to leverage its support in Congress to get more out of the White House.
This article was originally published on NBCNews.com