When Ukrainian President Volodymyr Zelenskyy lands in Washington Wednesday, he will come with one of his biggest requests of the U.S. since Russia invaded his country nearly 300 days ago — and he might not get the answer he wants.

Zelenskyy’s visit to the U.S., where he is set to meet President Joe Biden and to address members of Congress, came as a shock to senior U.S. officials and lawmakers alike. While there were whispers of a big event unfolding in Washington, none knew the exact details of the Ukrainian president’s arrival or plans.

Now that the in-person meeting is confirmed, officials say to expect a standoff over weapons and an attempt by both Biden and Zelenskyy to use the meeting to woo the incoming Congress.

Behind the scenes, U.S. and Ukrainian officials had been planning the visit for weeks — just as Washington and Kyiv began to face off in one of their tensest back-and-forths about the future of the war since the fighting began nearly 10 months ago. On a Dec. 11 call, Biden invited Zelenskyy to Washington to talk through forthcoming battlefield operations and ways his administration could continue supporting Ukraine in its fight against Russia, a senior administration official told reporters Tuesday.

In the weeks leading up to that call, U.S. officials had been engaged in talks with their counterparts in Kyiv and in Europe about how much military support the U.S. would continue to provide to Ukraine to aid its fight through the winter. Kyiv has appealed directly to the Biden administration for more advanced weapons — weapons that they say could potentially lead the way to victory. But the U.S. is hesitating, frustrating Ukrainian officials at a time when they are trying to advance on the battlefield and continue to push back Russian forces.

That conversation is expected to dominate the in-person meeting between Zelenskyy, his team and the Biden administration. Zelenskyy is also likely to make the case to Congress that his country is in need of more advanced weapons to fend off Russian bombardment.

“He needs longer-range weapons to enable him to preempt an expected Russian offensive,” said Bill Taylor, the former U.S. ambassador to Ukraine. “He needs immediate financial support to keep his government running, and he knows that Congress will make decisions on these items this week.”

While the Biden administration has vowed to continue supporting Kyiv in its efforts to combat Russian forces inside Ukraine, U.S. officials have resisted supplying any resources that would allow Ukrainians to launch long-range missiles at Russian positions inside Russia — weapons Ukrainians say are critical for allowing them to gain back lost territory and force Russia on the defensive.

During the meeting with Biden and his national security team, the Ukrainian delegation is expected to make another round of pleas for long-range Army Tactical Missile Systems, or ATACMS, and Grey Eagle and Reaper drones, according to a person familiar with the discussions. The person and others were granted anonymity to describe internal discussions.

But U.S. officials aren’t ready to budge. The Biden White House has flatly rejected sending the ATACMS. The costs of doing so are high, U.S. officials say. Sending long-range missiles to Kyiv could risk provoking Putin using potentially even more lethal weapons inside Ukraine.

That U.S. hesitation has concerned Ukrainian officials and forced senior military leaders inside the country to seek help from others, including countries outside the Western alliance. Now, the Ukrainians fear they will not be able to advance on the battlefield this winter and will lose the momentum they gained after liberating Kherson.

“For us the strategy is to continue attacking because we cannot afford to freeze the frontline. We need to constantly press,” said Oleksandr Danylyuk, Ukraine’s former national security adviser, who has continued to advise on military planning. “We have reached the limit of what we can do with the advanced weapons the U.S. has already provided. For the next stage, we need the longer-range weapons to achieve [our] goals.”

Since the war began, the Ukrainian government has repeatedly requested more advanced weapons. And the U.S. and other Western allies have moved — albeit slowly — to fill such requests, working to evaluate their potential effectiveness on the battlefield and whether they required additional training and special transport.

On Wednesday, Biden plans to announce that it is giving Ukraine more of the other weapons it has asked for, even as it stands firm against supplying longer-range resources.

The administration will present a new, $2 billion military aid package that will for the first time include a Patriot air defense battery and Joint Direct Attack Munition kits — which turn fighter jets’ unguided bombs into precision-guided munitions, said said the person familiar with the discussions. Also included in the package is $850 million worth of artillery and other munitions from existing U.S. stockpiles.

The shipment of the Patriot is a decision that surprised even Ukrainian officials. The Patriot system is a long-sought prize for Kyiv and the version headed to Ukraine is built to intercept ballistic missiles. That would be a major addition to Kyiv’s defenses against continued Russian attacks on the capital’s critical infrastructure.

But Ukraine also desperately wants to make aggressive advances in the eastern part of the country before they lose too much momentum and officials in Kyiv say the promised weapons aren’t enough for that.

Biden administration officials agree that Ukraine is at a precarious moment. The situation on the ground is likely to grow more dire this winter, with Russia launching more missiles at Ukraine’s military and critical infrastructure, a senior U.S. official said, pushing more civilians to flee to other regions of Ukraine and to nearby countries.

While the U.S. has ramped up its support to help Ukraine patch its energy grids, power continues to go out for long stretches of time in the country’s main cities. Russia also continues to rely on new shipments of Iranian drones, that U.S. official said, which will require Ukrainians to shoot down as many as possible to preserve their energy infrastructure.

The Biden administration will likely try to use the visit to underscore its more general support for Kyiv and its intention to continue supporting Ukraine through what are likely to be some of the darkest days of the war.

“This support is not just about what we have done before but what we will do today and what we will continue to do for as long as it takes,” the senior administration official told reporters Tuesday. “We know that the days ahead, the conflict will continue. The winter will be hard and we will continue day in day out to provide critical support to the Ukrainian people.”

U.S. officials say highlighting the administration’s support for Ukraine at this point in the war is critical for continuing to gain buy-in from Western allies in Europe, which have been strained by an energy crisis and fears of recession.

Zelenskyy’s visit, White House aides believe, would be a powerful message to the rest of the alliance across the Atlantic.

But the primary audience for Biden is much closer to home.

“As we head deeper into winter, as we continue to provide Ukraine the capabilities and needs on the battlefield, as we continue to surge diplomatic support and solidarity to Ukraine — this moment felt like a good opportunity for them to be able to have this engagement … and for President Zelenskyy to have this opportunity to address the American people,” the senior administration official told reporters.

Though publicly downplaying their concerns, White House aides have grown worried about the impact of the impending Republican takeover of Congress on funding to Kyiv. Kevin McCarthy, the possible next House Speaker, has vowed to no longer give Ukraine “a blank check.” And several influential far right Republicans – including Reps. Marjorie Taylor Greene (Ga.) and Lauren Boebert (Colo.) — have vowed to stop Washington’s assistance.

In a briefing with lawmakers last week, senior national security officials said more funding was needed to continue supporting Ukraine’s battlefield operations through the winter, according to a person directly familiar with the briefing.

Though the Biden administration does not believe that the aid will completely cease, officials privately acknowledge that there is concern that the amount of funding will likely shrink and slow down in the coming months. A Zelenskyy visit, they believe, could act as a bulwark. And Biden, with renewed political strength after a run of legislative success and a triumphant midterm election, has pushed to make Ukraine center stage as the year draws to a close.

Many Western leaders have covertly slipped into Kyiv since the war began, but U.S. officials have been deeply wary of transporting Biden into a war zone, for fears over the president’s security and of provoking Putin.

There had also been some chatter that the men could meet if Zelenskyy attended the G-20 in Indonesia last month, but that trip never materialized. But another idea took hold in recent months that would allow Biden to fully express solidarity with the unlikely Ukrainian wartime leader: a visit to Washington.

Nahal Toosi and Sarah Ferris contributed to this report