There's concern that success for Trump supporters in Tuesday's vote may jeopardise aid for Ukraine, damage transatlantic ties, and galvanise Europe's far-right.
America has been voting on Tuesday in midterm elections that have divided the country.
And in Europe, there are concerns that a strong showing by the right-wing Republican party -- and in particular some of the more extreme candidates endorsed by former President Donald Trump -- could send some rather unwelcome shockwaves across the Atlantic.
If the Republicans win control of Congress, Joe Biden's presidency will be knocked for six. Serious questions are being asked about the impact on US support for Ukraine against Russia's war, European security, and transatlantic trade ties.
Amid a toxic political atmosphere and with the last US president indicating he wants to run again in 2024, a Trump-turbocharged Republican Party would surely galvanise the populist right in Europe and elsewhere.
'A ballot for US democracy'
The economy, and especially inflation, looms large for voters -- with divisions also running deep on issues such as abortion rights, crime, immigration, gun control and climate change.
US midterm elections traditionally deliver sitting presidents and congressional majorities a bloody nose. Despite some noticeable legislative achievements, Biden's approval ratings are particularly low.
This time, conventional battlegrounds are increasingly tainted by disinformation and conspiracy theories, the threat of political violence, and faith in democracy itself.
"This is not simply a Democrat versus Republican election, it's also a ballot for control of the Republican party, which is in a deep, deep fight for what it stands for," said Professor Scott Lucas of the UK's Birmingham University and University College Dublin, and editor of EA Worldview.
"Ultimately this is a ballot for American democracy," he told Euronews. "The guardrails for American democracy are being taken down."
"The system just about held in 2020/21," he added, because the institutions ensured that Trump's "attempted coup" in trying to overturn the election failed.
But the great lie that the vote was "stolen" is still very much alive: hundreds of so-called "election deniers" who peddle Trump's baseless claims are running for office, both in Congress and at state level.
"One of the undercover stories of this election is the attempt by election deniers to win positions such as secretaries of state of the individual states, which has a huge impact in the way the elections would be run and monitored in 2024," Lucas added.
Is US aid for Ukraine at risk?
To date, the lion's share of Western allies' financial support for Ukraine since Russia launched its war in February has been borne by Washington. The value of total US commitments tops €52 billion, according to the Kiel Institute for the World Economy.
But there are fears that support could wane -- not just for Ukraine but for wider European security -- especially if Republicans take control of Congress.
"I think people are gonna be sitting in a recession and they’re not going to write a blank check to Ukraine," US House of Representatives Minority Leader Kevin McCarthy -- a strong supporter of Ukraine -- said in a recent interview.
In May, 57 House Republicans and 11 Republican Senators voted against a major aid package for Ukraine worth €40 billion.
A Reuters/Ipsos opinion poll in October said nearly three-quarters of Americans thought the United States should continue to support Ukraine. A Chicago Council Survey in August said 58% of Americans were willing to continue backing the country "for as long as it takes".
A Pew Research Center survey in September said that Republicans were more likely to say the US was providing too much support for Ukraine than too little -- although they represented only a third of the group sampled.
US political scientist Professor David Schultz told Lithuania's Mykolas Romeris University that he feared US policy could change if control of Congress shifted to the right.
"A Republican House with many Trump supporters may follow the former president’s lead and be less supportive of US assistance to Europe, Lithuania, or Ukraine. It may be more difficult for President Biden to get money for Ukraine, or to rally congressional support for further actions, if the war escalates," he argued.
However Max Bergmann, director of the Europe Program at the Center for Strategic and International Studies in Washington, argues that future aid is likely to be subject to trade-offs in bargaining between Republicans and Democrats, rather than blocked altogether.
"I do think overall there is strong bi-partisan support, and if the chips come down, and if Ukraine is (saying) 'we need 10 billion more to sustain this', I think the United States will be able to find it, whether from Congress or by re-allocating funding within the Pentagon," he told a Politico podcast.
Trade and competition: more 'America First'?
Relations between Washington and Brussels have certainly thawed under Joe Biden's time in office compared to that of his predecessor. But the warm words and unity on Ukraine belie underlying tensions.
There is frustration in EU circles that the president has not moved further from Donald Trump on trade and competition, and that in some respects the US has continued to pursue the "America First" agenda.
Legislation to curb inflation and promote clean energy includes incentives for US-made electric vehicles that are not extended to European manufacturers, causing anger in Europe.
"I was struck by a pervasive anxiety regarding the United States, particularly if polls predicting a Republican win in the midterm elections prove accurate," Ivo Daalder, former US ambassador to NATO, reported on Monday after a week-long tour of major European capitals.
"Europeans are right to feel uneasy about the direction of US trade and foreign economic policy... rather than cooperating, so as to compete more effectively with China, US policies are seen as pitting American companies against those in Europe and Asia," he wrote, calling for Washington and its allies to work together to face global challenges.
Green light for Europe's far right?
Beyond the Republicans' midterm performance, many will be watching for confirmation from Donald Trump of another presidential bid -- and a renewed populist drive to inspire his admirers the world over.
The defeated president and his allies have continued courting links with authoritarian leaders and sympathisers abroad in a bid to boost his brand.
None figures more strongly than Hungary's Viktor Orban -- who in August was invited to Trump's New Jersey estate and addressed a Conservative Political Action Conference in Texas, calling on conservatives in the US and Europe to take back power from liberals.
Budapest -- still tussling with the EU over "democratic backsliding" -- has openly called for Trump's return to power, an official government tweet saying "here's to hoping we will get there once more".
Another regular speaker at CPAC rallies is the UK's Nigel Farage, hugely influential in setting the Brexit ball rolling. He told the conference in August that the biggest threat to the West was not Putin but a coalition of mainstream media and educational "marxists".
US conservatives were lucky to have a man with the "courage" of Donald Trump to fight the "globalists" and "deep state", Farage said.
France's Marine Le Pen -- who in 2017 cited Trump and Vladimir Putin as her two political guiding lights -- has since cooled in her assessment of the ex-president, saying last February that he "no longer represents an active political force".
She condemned the Capitol insurrection of 6 January 2021 -- suggesting however that Trump had "underestimated" the impact of his words -- but did not acknowledge Biden's victory in the presidential election until the day afterwards, two months after the vote itself. And many of her National Rally supporters openly cheered the assault on Congress.
Professor Scott Lucas worries that amid the polarised electorate in the US midterm elections, people care less about the issues and "more about taking up emotive positions," rallying around their political tribe -- and in the case of many Republicans, constantly crying foul over alleged electoral fraud.
"The danger of that emotive performance is that this is precisely what allows the Trumpists to at least control the news cycle or control the social space with these unfounded claims, these allegations which despite every time that we knock them back, they simply get repeated," he told Euronews.
"Until you change the discussion or refocus the discussion on what is really important here, we're going to be playing out this Kabuki play, and possibly watching the American system crumble while it happens."