WASHINGTON - When he made his surprise wartime trip to the Ukrainian capital of Kyiv last week, United States President Joe Biden reassured that country with great confidence that “the Americans stand with you”.

But the question that remains unanswered is: For how long?

For all of the President’s bravado while he was abroad, the politics of Ukraine back home in the US are shifting noticeably and, for the White House, worryingly.

Polls show public support for arming the Ukrainians softening, while the two leading Republican presidential candidates are increasingly speaking out against involvement in the war.

While the bipartisan coalition in Congress favouring Ukraine has been strong in the year since Russia’s invasion, supporters of more aid fear the centrifugal forces of the emerging presidential contest and growing taxpayer fatigue with shipping tens of billions of dollars overseas may undercut the war effort before Moscow can be defeated.

And some of them are frustrated that Mr Biden has not done more to shore up support.

The evolving dynamics were on full display this week when House Republicans, exercising the power of their new majority, pressed Pentagon officials at two hearings about spending on Ukraine, grilling them about where the money is going and vowing to hold them accountable.

Despite Mr Biden’s pledge, the Ukrainian government has grown concerned enough that President Volodymyr Zelensky is trying to set up a telephone call with Speaker Kevin McCarthy to make his country’s case.

Overall, public support for Ukraine aid has fallen from 60 per cent last May to 48 per cent now, according to surveys by The Associated Press-NORC Centre for Public Affairs Research.

The share of Americans who think the US has given too much to Ukraine has grown from 7 per cent a year ago to 26 per cent last month, according to the Pew Research Center.

And even supporters make clear their commitment is not without bounds.

While 50 per cent of those surveyed by Fox News said US support should continue for “as long as it takes to win”, 46 per cent said the time frame should be limited.

“It’s this way with every foreign intervention,” said Mr Andy Surabian, a Republican strategist who has advised two outspoken Republican voices against Ukraine aid, Senator J. D. Vance of Ohio and Mr Donald Trump Jr.

“In the first few months, it’s always popular. People don’t like what Russia did; it’s awful. But as time goes on, war weariness is a real thing, especially in this country, especially when voters aren’t connecting what’s happening in Ukraine with their own security.”

Although scepticism of Ukraine aid has grown on both sides of the aisle, the party breakdown has been striking.

According to Pew, 40 per cent of Republicans think too much has been given compared with 15 per cent of Democrats.

The good news for Mr Biden is that Americans have grown more supportive of his handling of the war, with 48 per cent approving of his response to the invasion in the Fox poll, compared with 40 per cent in August.

While Mr Biden used his visit to Kyiv and a follow-up stop in Warsaw, Poland, to express solidarity with the Ukrainians, he has talked less about the war to fellow Americans while at home.

He made a relatively passing reference to the war during his State of the Union address and has focused mainly on domestic priorities in recent campaign-style stops around the country.

In part, that may be intended to deflect criticism that he cares more about foreigners than Americans.

Aides said Mr Biden’s speeches in Kyiv and Warsaw were intended for an American audience as well as international ones.

But the President has shrugged off concerns about ebbing public support for the Ukraine supply effort, suggesting it is relegated mainly to what he calls Maga Republicans, after former president Donald Trump’s Make America Great Again slogan.

Mr John Kirby, a spokesman for the National Security Council, said support remains powerful in Congress itself.

“Yes, there are a small number of members on Capitol Hill, in the House Republicans specifically, that have expressed publicly their concerns about support for Ukraine,” he said at a recent briefing.

“But if you talk to the House leadership, you won’t hear that. And you certainly aren’t going to hear it on the Democratic side. And you don’t hear it in the Senate.”

Indeed, Senator Mitch McConnell of Kentucky, the Republican leader, and key House Republicans such as Representative Michael McCaul of Texas, chair of the Foreign Affairs Committee, have pushed Mr Biden from the other side, arguing that the President is not doing enough for Ukraine.

Mr McCaul took a congressional delegation to Kyiv shortly after Mr Biden, emphasising bipartisan support.

But Mr McCarthy, who during last fall’s campaign said there would be no “blank cheque” for Ukraine in a Republican House, is under pressure from a small but vocal part of his caucus critical of the US’ involvement in the war and encouraged by Fox News host Tucker Carlson.

With a razor-thin working majority, it is not clear whether he would allow another robust aid package to come to the floor for a vote and, if so, under what conditions, which is why Mr Zelensky wants to talk, as was reported by Punchbowl News.

Among those pushing Mr McCarthy to block future aid is Representative Marjorie Taylor Greene, a former QAnon adherent who has become a key ally since helping him win the speakership.

Speaking to Just the News, a conservative website, this week, Ms Greene said she opposed the war in Ukraine.

“But you know who’s driving it?” she asked. “It’s America. America needs to stop pushing the war in Ukraine.”

While she and her allies have been on the margins of the Republican Party on Ukraine, the centre of gravity may be shifting.

Mr Trump lashed out at Mr Biden last week for visiting Kyiv instead of East Palestine, Ohio, the site of a recent toxic train derailment.

In a fund-raising video, Mr Trump said “we’re teetering on the brink of World War III” thanks to Mr Biden, and promised to “end the Ukraine conflict in 24 hours”.

Florida Governor Ron DeSantis, his most formidable potential challenger for the 2024 nomination, sought to match Mr Trump, criticising what he called the “open-ended blank cheque” for Ukraine and saying: “I don’t think it’s in our interest” to be involved in the fight for territory seized by Russia.

By contrast, the announced and unannounced Republican presidential candidates who do support aid to Ukraine, like former vice-president Mike Pence and Ms Nikki Haley, former ambassador to the United Nations, trail far behind those two front runners.

So far, Congress has approved US$113 billion (S$151.94 billion) in military, economic, humanitarian and other aid for Ukraine, not all of which has been spent.

Anticipating trouble from the new Republican House, the White House and Democratic majority last winter pushed through an aid package large enough to last until summer.

At the current rate of spending, it would run out by mid-July, according to the Centre for Strategic and International Studies.

A House Democrat who asked not to be identified speaking critically of the White House expressed concern that the President’s team did not fully grasp how Americans viewed the aid.

While they support Ukraine in principle, this Democrat said, the way the aid has been doled out through a steady drumbeat of announcements of another US$500 million or US$1 billion every week or two exacerbates the sense that endless funds are heading out of the country.

Mr Philip Zelikow, a University of Virginia scholar and former State Department counsellor, said military aid was more popular than economic aid because much of it is actually spent on arms produced by US defence firms.

But he said that economic aid was critical to rebuilding Ukraine, and he argued that seizing US$300 billion in Russian assets in the West for reconstruction would ease the burden on the American taxpayer.

“I’m critical of the administration because it did not start moving at least six months ago to design a more sustainable and hopeful strategy on what will likely be the decisive battlefield of the war,” he said.

Still, some government veterans said there is only so much Mr Biden can do to preserve public support, since the most pronounced erosion has been on the Republican side.

“President Biden probably has limited ability to reach the Republican audiences that are most in play,” said Professor Peter Feaver, a Duke University academic who has studied the relationship between public opinion and military operations and advised President George W. Bush during the Iraq War.

“He has a daunting but perhaps doable task to keep his left flank on board.” NYTIMES