President Joe Biden will ask Congress for billions of dollars of funding for Israel and Ukraine in a primetime address at the White House on Thursday.
The bulk of the $100bn (£82bn) request is expected to cover military aid for Israel and Ukraine, with some money for Taiwan and US border security.
But spending measures cannot currently pass the US House of Representatives because it does not have a Speaker.
The president’s speech comes a day after his whirlwind trip to Israel.
Mr Biden will give only his second address to the nation from the Oval Office at 20:00 local time (midnight GMT).
Of the $100bn request, congressional aides told BBC News they expect $10bn would go to Israel, at least $60bn to Ukraine and the remainder be divided between Taiwan and boosting security at the southern border of the United States.
Some Republican lawmakers have stalled funding efforts for Ukraine, as it becomes a growing political cudgel on the right.
Democrats hope the request can receive bipartisan support by tying funding for Ukraine to other issues – such as US border security – that are Republican priorities.
A senior Democratic staffer in the House of Representatives told BBC News the aim is for the bipartisan coalition that averted a government shutdown last month to co-operate on the proposed aid package.
The Republican deal with Democrats that staved off a government shutdown, however, led to House Speaker Kevin McCarthy being ousted in a right-wing revolt on 3 October.
Congress remains paralysed as Republicans have failed to elect a new Speaker, leaving any legislation, including all spending requests, shelved for now.
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The current nominee for Speaker, a right-wing Ohio congressman Jim Jordan, has previously expressed opposition to further Ukraine aid.
Supporters of Mr Biden’s funding request believe it stands a better chance through interim Speaker Patrick McHenry, who negotiated last month’s deal with Democrats to temporarily fund the government.
“Some of the hawks don’t think Jordan has any real experience in national security,” a Republican staffer on the Senate Armed Services Committee said. “And he’s proved hostile to Ukraine.”
The Republican staffer said that Israel has asked for specific capabilities: precision-guided munitions and arms to replenish its Iron Dome air defence system.
The anticipated request would be received favourably by Republican senators, especially if border security funding is included, the staffer said.
But there is Republican opposition to the request, even before its official announcement.
“I do believe Israel’s asked for $10bn, so a $100bn request is ridiculous,” Andy Ogles, a hardline Republican in the House, told reporters on Thursday. “They should be separated.”
Mitch McConnell, the Republican leader in the Senate, has shown willingness to take up the measure.
In a speech on the Senate floor on Thursday, the Kentucky legislator said it was “naive and dangerous” to oppose sending military aid to American allies.
He said Israel, Ukraine and Taiwan have “no choice” but to stand up to the respective threats they face from Hamas, Russia and China.
The Republican staffer said it would probably take Congress “a week or two” to review the request before voting.
The hope is that lawmakers will “get it done prior to Thanksgiving” at the end of November.
But it all may depend on whether the House, which controls America’s purse strings, finally resolves its leadership issues.