Just two days later, the Republican Speaker of the U.S. House of Representatives, Mike Johnson, who has so far blocked passage of a bill that includes $60 billion in new funding for Ukraine, posted a picture of himself smiling with former Republican President Donald Trump, who has opposed aid for Kyiv.

The contrast underscored the challenges facing the Biden administration if Congress fails to approve more military assistance, which Ukraine desperately needs to hold off Russian invaders.

So far, President Joe Biden's administration has ruled out discussing a plan B.

Trump, frontrunner for the Republican presidential nomination and a long-standing critic of the NATO alliance, has in recent weeks threatened to abandon some European allies if they were to be attacked by Russia.

As Vice President Kamala Harris and other administration officials projected confidence last week at a Western security gathering in Munich, Kyiv was losing territory to Russia. Moscow took control on Sunday of the town of Avdiivka, its biggest gain in the past nine months.

"This happened in large part because Ukraine was running out of weapons due to congressional inaction," Biden National Security Adviser Jake Sullivan said on Tuesday, warning of worse to come if lawmakers do not act.

The Senate last week approved a $95 billion bill providing assistance for Ukraine, Israel and Taiwan by an overwhelming 70-30 vote, with 22 Republicans joining most Democrats in voting "aye." But Johnson sent the House home for a two-week recess without bringing the measure up for a vote.

Since then, Senate Republicans and Democrats have joined those urging passage of the aid.

If approved, the funding would bring the total U.S. investment in the conflict to $170 billion, although Congress has not approved any major aid for Ukraine since Republicans took control of the House in January 2023.

Nearly two-thirds of the $60 billion would go to U.S. companies that make military equipment for Ukraine, much of it to replace materiel already sent east.

TWO WEEKS WAIT, AT LEAST

The House is unlikely to consider security assistance before mid-March.

Republican House members have said they do not want to take up the broad $95 billion national security supplemental as it is, although members of both parties acknowledge it would pass easily if Johnson allowed a vote.

Johnson voted repeatedly against aid for Ukraine before he became House speaker last year.

"At the end of the day, I do think there's still a majority in the House that will pass this, it just has to come to the floor," Representative Michael McCaul, the Republican chairman of the House Foreign Affairs Committee and a backer of Ukraine aid, recently told reporters.

Johnson has suggested he wants to break the security aid legislation into separate bills for Ukraine and Israel, although two previous House bills providing aid only to Israel have failed.

Changes would further delay aid for Ukraine as any new measure passed by the House must also be approved by the Senate before it can be sent to the White House for Biden's signature.

Other Republicans suggested the House might amend the Senate bill. McCaul said one possibility was adding a provision - known as the REPO Act - to confiscate Russian assets and hand them to Ukraine.

Another is converting economic assistance for Ukraine from a grant to a loan, something backed by Trump.

Some House members have threatened to use a procedural tool known as a discharge petition to get around the Speaker. That would need several House Republicans to oppose their leader, so even Ukraine supporters said they did not expect such an effort before other options were exhausted.

PLAN B?

If Congress fails to act, Biden appears to have few good options for supporting Ukraine.

The administration has so far refused to discuss any "Plan B", instead focusing on the supplemental.

"There is no magic solution to this, absent Congress appropriating funding," Sullivan said.

One U.S. official said the administration was "doing everything it can" to meet immediate contingencies and ensure that the assistance Washington is providing helps Ukraine address shortages on the battlefield.

Jeremy Shapiro, director of research at the European Council of Foreign Relations, said if the supplemental fails, the administration could use various executive powers to help Kyiv.

"They're going to look to shift money from one place to another. They're going to look for ways to send other monies or equipment to other allies who will then ship it on onto Ukrainians," Shapiro said.

"It will certainly make this less efficient and they'll be able to spend less," he added.