Trump has threatened to upend NATO, cut a quick deal with Russia over Ukraine and impose trade tariffs if he wins November's presidential election.
Harris has stressed the importance of NATO and decried Trump's tariffs as a hidden sales tax.
It is no secret that most European officials would prefer the more predictable Democratic vice president rather than the volatile Republican take the reins at such a perilous moment.
Ahead of the ballot, here are some of the biggest worries for Europe:
NATO
Trump unleashed a firestorm on the campaign trail by saying he would encourage Russia to do "whatever the hell" it wants with NATO members not spending enough on defence.
The broadside was the latest from the ex-reality TV star to rattle the alliance that has underpinned Europe's security for 75 years.
Having already weathered four years of Trump in charge, NATO diplomats insist the nightmare prospect of Washington pulling out remains distant.
To head off his criticism, European nations have played up their increased defence spending since Moscow's 2022 invasion of Ukraine.
This year 23 of NATO's 32 members are set to hit the target of 2 percent of GDP -- up from three a decade ago.
"It's a Trumpian way of doing business, it's the way he says it, but it's a totally reasonable message on the necessity for Europe to do more," said one European diplomat.
If Harris wins, the tone towards NATO is set to be notably warmer. She has echoed President Joe Biden's pledge to stand strong with US allies.
But even under Harris, European countries expect major pressure to keep increasing defence spending.
And as the United States increasingly focuses on its rivalry with China, there is a sense that either way Europe will have to take more responsibility for its security in the long run.
Ukraine
While European officials are hopeful they could talk Trump down from undermining NATO, there are more questions swirling over US backing for Ukraine.
The United States and EU nations have given the bulk of support for Kyiv in a fight many in Europe see as an existential struggle for the continent's security.
While European countries combined have provided some $130 billion, Washington alone has come up with $90 billion, according to a tracker from the Kiel Institute.
Former president Trump has cast doubt on US help for Ukraine and insists he could do a deal with Russian leader Vladimir Putin to end the war in one day.
European officials say that should Trump pull the plug they would try to keep Ukraine afloat, but that would likely only keep Kyiv going until it was forced to accept a deal.
"But I'm not sure Trump will withdraw. He could call Putin, not get the deal he wants and decide to fully support Ukraine, said one diplomat. "You never know with him."
Like Biden, Harris has pledged to maintain Washington's firm backing for Kyiv.
But European diplomats say that she likely will not be as personally invested as Biden, and as the war drags on into a third year calls to end it somehow will grow.
Trade
The EU believes this time it will be battle-ready if Trump wins and follows through on threats to impose tariffs on imports to the United States.
The European Commission, the EU's executive arm in charge of trade policy, has set up a team to ready plans for a Harris or Trump win.
"The EU is much better prepared for another Trump administration," an EU diplomat said.
The team is however preparing for different scenarios and "unlike last time, we have various trade tools", the diplomat said.
There has been much talk of a "list" of US products the EU could slap tariffs on, but European officials and diplomats insist this would be a last-resort measure.
The first test for the EU and the United States under any new administration will be in March, when a truce relating to US steel and aluminium tariffs expires.
Trump in 2018 slapped high duties on steel and aluminium coming from many countries, sweeping up EU exports in the process. When Biden came to power, he kept the tariffs in place but granted suspensions for EU exporters.
Brussels fears Trump will impose higher tariffs while Harris, although unlikely to be soft, would maintain "dialogue in good faith", the EU diplomat said.
"There'll be a continued focus on trying to find those areas of cooperation, trying to avoid unnecessary conflict under a Harris administration," said Greta Peisch, a former General Counsel for the US Trade Representative.
Peisch, now a partner at US law firm Wiley, told AFP Harris would represent a "continuation" of Biden's administration, noting that under Trump, it could be more complicated.
"I think that there were areas of cooperation, but in the Trump administration, there was also more of a willingness to be confrontational on trade issues."
del-raz/gv
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