Since Russia’s invasion of Ukraine, President Joe Biden and Ukrainian President Volodymyr Zelenskyy have spoken by phone whenever the US announces a new package of military aid for Kyiv.

According to four people familiar with the call, a phone call between the two leaders in June went differently than previous ones. Biden had barely finished telling Zelenskyy that he had just approved another $1 billion in US military assistance for Ukraine when Zelenskyy began listing all the additional assistance, he required but was not receiving. People familiar with the call said Biden lost his cool. The American people were being extremely generous, and his administration and the US military were working tirelessly to assist Ukraine, he said, raising his voice, and Zelenskyy could show a little more gratitude.

According to administration officials, Biden and Zelenskyy’s relationship has only improved since their phone call in June, following which Zelenskyy issued a statement thanking the United States for its generous assistance. However, the spat reflects Biden’s early recognition that congressional and public support for sending billions of dollars to Ukraine could begin to dwindle. That moment has arrived, just as the president is preparing to ask Congress for additional funding for Ukraine.

Biden is now facing opposition from both Republicans and Democrats, which was not present when Congress approved previous Ukraine funding. The White House has discussed requesting billions of dollars from Congress during the lame duck legislative session following the midterm elections.

According to a source familiar with the conversation, Biden was direct with Zelenskyy about handling the issues through the proper military channels, but the exchange was not heated or angry.

According to three people familiar with the call, the president’s frustrations with Zelenskyy had been building for weeks prior to the June 15 phone call. Biden and some of his top aides believed that the administration was doing everything possible in a timely manner, but Zelenskyy continued to focus publicly on what wasn’t being done.

According to two sources familiar with the Ukraine government’s view, congressional aides and two European offices, 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 Zelenskyy’s perspective, as well as that of some Eastern European governments and US lawmakers from both parties.

The Ukrainian military is concentrating its efforts on driving thousands of Russian troops away from Kherson, attempting to encircle them and retake control of the southern city. The battle for Kherson may turn out to be one of the most important battles in Ukraine since the invasion. If Ukraine is able to retake the area, it will be a huge morale boost for Zelenskyy’s forces and a major setback for Russian troop confidence. However, if Russia holds on, it may be able to keep its grip on the south, including the Zaporizhzhia nuclear power plant, through the winter. “This could be a watershed moment,” said a defense official.

Concerns about dwindling support for Ukraine are also driving the current offensives, according to a defense official and a former official, as Ukraine attempts to demonstrate battlefield momentum in order to encourage the flow of more weapons.

On Oct. 12, Defense Secretary Lloyd Austin convened the Ukraine Contact Group, a regular gathering of allies, in Brussels to discuss how to get more weapons and equipment into Ukrainian military hands. While previous meetings have resulted in assistance ranging from ammunition to missile launchers, three defense officials familiar with the discussions said this month’s meeting took on new urgency.

Ukraine still requires more air defense systems to defend against Russian military aircraft, missiles, and drones, and the US is considering providing longer-range missile systems, such as the ATACMS, as well as advanced fighter aircraft in the future.

The potential shift in US political will to continue sending aid to Ukraine could upend how the White House and Zelenskyy have approached the issue thus far.

The shifting dynamics on Capitol Hill may also force Zelenskyy’s team to reconsider its approach to Washington, as it has frequently tried to leverage its support in Congress to get more from the White House.