File Photo: House Speaker Nancy Pelosi and Vice President Harris hold up a Ukrainian flag while Ukrainian President Volodymyr Zelenskyy speaks to Congress. AFP
Zelenskyy will appear via videolink during a classified briefing on the conflict, a day before the Senate is scheduled to take the first procedural vote on an emergency aid package that includes more than $60 billion for Kyiv.
The cash has been held up for weeks by a row over domestic policy in Congress, as the White House has warned that existing funds will run out by the end of the year and that Russia's President Vladimir Putin could win the war if lawmakers fail to act.
"We can't ever put a price on defending democracy in its hour of need, because if Ukraine falls, Putin will keep on going," Senate Majority Leader Chuck Schumer said as he announced Zelenskyy's address in the Senate late Monday.
"Autocrats around the world will be emboldened. Democracy, this grand and noble experiment, will enter an era of decline."
Underlining the stakes for Washington, Kyiv and beyond, several top Ukrainian officials were making the rounds in the US capital in person, alongside the speaker of the country's parliament, as part of a coordinated lobbying effort.
Top US administration officials including Secretary of State Antony Blinken, Defense Secretary Lloyd Austin and Director of National Intelligence Avril Haines were due to take part in the closed-door briefing.
Congress is more divided over backing for Ukraine than it has been at any time during the nearly two-year conflict.
Senate Republicans are conditioning their support for the funding on President Joe Biden's Democrats accepting measures to address the migrant crisis at the southern border -- reforms the Democrats have already rejected as "extreme."
In the latest setback, Democrats walked out of negotiations on changes to the asylum system and border security Friday after concluding that Republicans were refusing to compromise.
Schumer has teed up a vote Wednesday on clearing the first procedural hurdle for addressing Biden's $106 billion aid request for Ukraine, Israel and Taiwan.
But it needs 60 votes in the 100-member Senate, and the 49-strong Republican minority looks likely to sink the package as it leaves out their immigration reforms.
Texas Republican John Cornyn said in a speech on the Senate floor the proposal had "zero chance of becoming law."
"Our security cannot come second to that of other countries around the world, our allies, even those like Ukraine and Israel," he said.
Even if the two sides manage to hammer out a deal in the Senate, it will be a much tougher sell for the Republican-led House, where conservatives have been more skeptical about funding Ukraine, and just as keen to leverage the issue to secure border reforms.
House Speaker Mike Johnson confirmed publicly for the first time in a letter to the White House Tuesday that his party will not pass Ukraine aid unless Congress enacts "transformative change to our nation's border security laws."
Andriy Yermak, a top aide to Zelenskyy, thanked Americans for their support so far in an appearance in Washington hours before the president's scheduled address, but added that more aid was needed to secure victory.
"It's very difficult for our people but Ukrainians are still very motivated," Yermak said at the US Institute of Peace. "Our people believe and are sure that we will win."