Senate Democrats are planning to make a last-ditch effort on Wednesday to salvage an aid bill for Ukraine and Israel, with Republicans expected to kill a version of the package that includes stringent border security measures that they had demanded be included.

Senator Chuck Schumer of New York, the majority leader, has told his Democratic colleagues that after a critical test vote set for early Wednesday afternoon, in which Republicans are expected to block the border and Ukraine package, he plans to quickly force a vote on a stand-alone bill that would send tens of billions of dollars in funding to Kyiv and Israel.

A bipartisan group of senators had spent months negotiating a compromise that paired a crackdown against migration into the United States with an emergency national security spending package that has been stalled for months.

But with Republicans balking at the immigration deal, the outcome of that vote was clear: It did not have the 60 votes it needed to advance. Anticipating its failure, Mr. Schumer told the White House this week that he had a Plan B: If Republicans scuttled the bipartisan agreement, he would immediately seek to push through the foreign aid without the border deal, according to a Democratic aide who spoke on the condition of anonymity to describe the discussions.

That set up Republicans to potentially vote twice in one day to block the emergency national security supplemental bill, which includes $60.1 billion in military assistance for Ukraine, $14.1 billion in security assistance for Israel and $10 billion in humanitarian aid for civilians of global crises, including Palestinians and Ukrainians. Mr. Schumer described that outcome as an embarrassing prospect for a party reeling from a series of defeats.

Senator Mitch McConnell of Kentucky, the minority leader, has been a vocal champion of funding for Ukraine.

“Make no mistake, a gauntlet has been thrown, and America needs to pick it up,” Mr. McConnell said this week in a speech on the Senate floor, discussing how critical it was to send funding to Ukraine. He has traveled to Kyiv to show continued support for the war effort and has long been more concerned with sending the foreign aid abroad than with passing any immigration package.

Mr. Schumer’s tactic could work if the national security spending package could muster 60 votes, which would require the support of at least 10 Republicans. If the bill were to pass the Senate, it would put pressure on Republican leaders to bring the bill to the floor in the House, where it faces strong headwinds given the opposition of right-wing lawmakers to sending additional assistance to Ukraine.

On Tuesday night, the House defeated legislation that Republicans had put forward to send $17.6 billion in military assistance to only Israel. Democrats lambasted the bill as a political ploy to undermine efforts to pass a broader foreign military aid bill that included Ukraine.

For Speaker Mike Johnson, there is pressure on both sides. Representative Marjorie Taylor Greene, Republican of Georgia, has threatened to oust him from the speakership if he brings to the floor any bill that includes funding for Ukraine.