When Vice President Kamala Harris flew to Germany for the Munich Security Conference last year, she made an unequivocal promise. “The United States,” she said, “will continue to support Ukraine, and we will do so for as long as it takes.”

When Ms. Harris returned to the same forum and took the same stage this past week, her message sounded similar but there was one important difference. “You have made clear that Europe will stand with Ukraine,” she told the gathered leaders, “and I will make clear President Joe Biden and I will stand with Ukraine.”

Not the United States this time, but she and Mr. Biden. It was a personal pledge that she could make on behalf of herself and her president, but she could not be so definitive about her country. For those watching for clues, it was a seemingly subtle shift in wording that spoke volumes.

Neither Mr. Biden nor Ms. Harris can promise with any degree of certainty anymore that America really is in the fight with Ukraine for the long haul. House Republicans are blocking $60 billion in security aid even as Ukrainian troops short of ammunition and weaponry just have had to withdraw from the city of Avdiivka. And an election less than nine months away could return to office former President Donald J. Trump, no friend of Ukraine or NATO but an open admirer of President Vladimir V. Putin of Russia.

Ms. Harris was not trying to shirk from the fight during her trip to Munich — quite the opposite, she was doing everything she could to reassure nervous Ukrainians and Europeans of her administration’s resolve. But the reality is that the political uncertainty back home has destabilized the multinational coalition backing Ukraine just days from the second anniversary of Mr. Putin’s invasion.

President Volodymyr Zelensky of Ukraine gave voice to the anxiety. “The key issue for us now is the preservation of principal American support,” he said at a joint news conference with Ms. Harris. “Ukraine and all our warriors need and await the respective positive vote regarding the assistance package, and I think everybody understands how much depends on this single voting procedure.”

Ms. Harris told him that there were still bipartisan majorities in both houses of Congress in favor of Ukraine aid, even though House Republicans were not permitting a vote. If the bill got to the House floor, she told him, she had no doubt it would pass, just as it already had in the Senate.

“We must be unwavering, and we cannot play political games,” she said. “Political gamesmanship has no role to play in what is fundamentally about the significance of standing with an ally as it endures an unprovoked aggression.”

She would not entertain, at least in public, the notion that the administration may need a Plan B. “There is only Plan A, which is to ensure that Ukraine receives what it needs,” she said.

But few if any in Munich had much confidence in Plan A anymore. The Europeans, who just passed their own aid package, have heard American guarantees for months only to find that nothing is so guaranteed after all.

On and off the record, White House officials all the way back to last summer expressed supreme confidence that the aid would be approved. As recently as December, they brushed off doubters as modern-day Cassandras. As recently as a few days ago, even, they still thought it was likely to pass.

But then Mr. Trump intervened, and they seemed caught off guard. They still publicly express optimism that the aid will eventually pass, as Mr. Biden did when he called Mr. Zelensky from Delaware to reinforce Ms. Harris’s message and said that “I’m confident we’re going to get that money,” as the president recounted to reporters afterward. But privately, the cockiness of a few months ago has turned into deep concern.

In her speech at the Munich Security Conference on Friday, Ms. Harris was more intent on addressing the audience back home than the leaders and diplomats in the room. She tried to make the case for why it was important to stick with Ukraine and stand up to Mr. Putin as Mr. Trump talks about encouraging Russia to attack NATO allies that do not pay their fair share.

“Imagine if America turned our back on Ukraine and abandoned our NATO allies and abandoned our treaty commitments,” she said. “Imagine if we went easy on Putin, let alone encouraged him. History offers a clue. If we stand by while an aggressor invades its neighbor with impunity, they will keep going.”

Her case was bolstered by a stunning turn of events. Just before she took the stage, word arrived that the Russian dissident Aleksei A. Navalny had died in one of Mr. Putin’s prisons, news that rippled through the Bayerischer Hof hotel where the conference was being held.

Nothing could do more to remind the audience of Mr. Putin’s ruthless rule, and Ms. Harris quickly added a condemnation to the top of her remarks. She was all but rushed off the stage after her speech, though, so that Yulia Navalnaya, the dissident’s wife, could make a dramatic surprise appearance condemning Mr. Putin and vowing to bring him to justice.

The death of Mr. Navalny prompted some hope among Biden administration officials that the shock would wake up House Republicans and force them to take action on the aid. They were heartened to hear that Speaker Mike Johnson had issued a statement saying that the United States and its allies “must be using every means available to cut off Putin’s ability to fund his unprovoked war in Ukraine.”

Like many in Washington, Ms. Harris has never met Mr. Johnson, the conservative backbencher from Louisiana who was abruptly elevated to the speakership on the back of a hard-line Republican rebellion a few months ago, and she was careful not to single him out for criticism in her public comments on Saturday.

But some officials, feeling burned by Republican flip-flopping on Ukraine, worried that they were reading too much into Mr. Johnson’s statement, especially given that the House has left Washington for a two-week recess. That means lawmakers will not return until after the initial shock of Mr. Navalny’s death has faded.

Perhaps less sanguine, Mr. Zelensky reminded the conference in his own speech that “dictators do not go on vacation.”

Ms. Harris’s meeting with Mr. Zelensky in Munich on Saturday brought the two back to where it started for them. They sat down in the same room of the same bank across the street from the conference hotel where they first met two years ago almost to the day — five days before the Russians marched across the Ukrainian border.

Back then, Ms. Harris was trying to persuade Mr. Zelensky to take American warnings of imminent Russian aggression seriously. This time she was left to deliver the message that America was not abandoning the effort no matter what the politics at home.

“You have shown extraordinary courage and accomplishment on the battlefield,” she told him on Saturday.

Mr. Zelensky, wearing a black sweater, appeared worn, the exhaustion of two years of war visible on his face. But he has learned since the early days to temper his approach to American benefactors, who were irritated at first that he never seemed grateful for all they had done and instead used meetings with the president and vice president to go over lists of specific military hardware he needed, the kind of details usually left to lower levels.

The Mr. Zelensky who appeared in Munich this time was a leader who recognized that the weapons flow was no longer a given, and he suffused his public and private comments on Saturday with plenty of appreciation.

“We are very thankful,” he said, “not only from me and my team, first of all, from all our people, are thankful to you, to people of the United States, your society, great society, and to President Biden, his team and of course bipartisan support, we are thankful for this.”

“But,” he quickly went on, “we need now your unity during such a challenging period for us.”

“And of course in the United States,” it too is a “challenging period,” he added. “We understand everything.”