LONDON--As chances rise of a Joe Biden-Donald Trump rematch in the U.S. presidential election race, America’s allies are bracing for a bumpy ride.

Many worry that a second term for Trump would be an earthquake, but tremors already abound — and concerns are rising that the U.S. could grow less dependable regardless of who wins. With a divided electorate and gridlock in Congress, the next American president could easily become consumed by manifold challenges at home--before even beginning to address flashpoints around the world from Ukraine to the Middle East.

French President Emmanuel Macron’s recent verdict was blunt: America’s “first priority is itself.”

The first Trump administration stress-tested the bonds between the U.S. and its allies, particularly in Europe. Trump derided the leaders of some friendly nations, including Germany’s Angela Merkel and Britain’s Theresa May, while praising authoritarians such as Turkish President Recep Tayyip Erdogan and Russian leader Vladimir Putin. He has called China’s Xi Jinping “brilliant” and Hungary’s Viktor Orbán “a great leader.”

In campaign speeches, Trump remains skeptical of organizations such as NATO, often lamenting the billions the U.S. spends on the military alliance whose support has been critical to Ukraine’s fight against Russia's invasion.

Biden, in contrast, has made support for Ukraine a key priority and moral imperative. But Biden’s assertion after his election in 2020 that “America is back” on the global stage has not been entirely borne out. Congressional Republicans have stalled more military aid for Ukraine, while America's influence has been unable to contain conflict in the Middle East.

Thomas Gift, director of the Centre on U.S. Politics at University College London, said that whoever wins the presidential race, the direction of travel will be the same – toward a multipolar planet in which the United States is no longer “the indisputable world superpower.”

Most allied leaders refrain from commenting directly on the U.S. election, sticking to the line that it’s for Americans to pick their leader.

They are conscious that they will have to work with the eventual winner, whoever it is — and behind the scenes, governments will be doing the “backroom work” of quietly establishing links with the contenders’ political teams, said Richard Dalton, a former senior British diplomat.

But many of America's European NATO allies are worried that with or without Trump, the U.S. is becoming less reliable. Some have started to talk openly about the need for members to ramp up military spending, and to plan for an alliance without the United States.

German Chancellor Olaf Scholz said he was “currently on the phone a lot with my colleagues and asking them to do more” to support Ukraine. Germany is the second-largest donor of military aid to Kyiv, behind the U.S., but Scholz recently told Die Zeit that the country couldn’t fill any gap on its own if “the U.S.A. ceased to be a supporter.”

Russia, meanwhile, is busy bolstering ties with China, Iran and North Korea and trying to chip away at Ukraine’s international support.

Macron also suggested American attention was focused far from Europe. If Washington's top priority is the U.S., he said its second is China.

“This is also why I want a stronger Europe, that knows how to protect itself and isn’t dependent on others,” Macron said at a January news conference.

Trump does have supporters in Europe, notably pro-Russia populists such as Hungary's Orbán. But former British Prime Minister Boris Johnson raised some eyebrows when he argued recently that “a Trump presidency could be just what the world needs.”

Johnson is a strong supporter of Ukraine in its struggle against Russian invasion, whereas Trump has frequently praised Putin and said he’d end the war within 24 hours. However, Johnson said in a Daily Mail column that he didn’t believe Trump would “ditch the Ukrainians,” but instead would help Ukraine win the war, leaving the West stronger “and the world more stable.”

Bronwen Maddox, director of the international affairs think tank Chatham House, said arguments like that underestimate “how destabilizing” Trump has been, and likely would continue to be if reelected.

“For those who say his first term did not do much damage to international order, one answer is that he took the U.S. out of the JCPOA, the deal to curb Iran’s nuclear program. Iran’s acceleration of its work since then has left it a threshold nuclear weapon state,” she said during a recent speech on the year ahead.

Biden was a critic of Trump's Iran policy but hasn't managed to rebuild bridges with Tehran, which continues to flex its muscles across the region.

Dalton, a former U.K. ambassador to Iran, said prospects for the Middle East would be “slightly worse” under Trump than Biden. But he said divergence on the region’s main tensions — the Israeli-Palestinian conflict and Iran's ambitions — would be limited.

“No U.S. administration is going to make a serious effort to resolve differences with Iran through diplomacy," Dalton told The Associated Press. "That ship sailed quite some time ago.”

Palestinians and their supporters, meanwhile, implore Biden to temper U.S. support for Israel as the civilian death toll from the war in Gaza climbs. But hard-liners in Israel argue the U.S. is already restraining the offensive against Hamas too much.

Itamar Ben-Gvir, Israel’s far-right national security minister, recently said Biden was not giving Israel his “full backing” and that “if Trump was in power, the U.S. conduct would be completely different.”

Much like its allies, America's rivals are not openly expressing a preference for the election outcome.

Trump developed a strong rapport with Turkey’s Erdogan, calling them “very good friends” during a 2019 meeting at the White House.

Yet Turkey-U.S. relations were fraught during his tenure. The Trump administration removed Turkey from its F-35 fighter jet project over Ankara’s decision to purchase Russian-made missile defense systems, while Trump himself threatened to ruin Turkey’s economy.

Russian Foreign Minister Sergey Lavrov told CBS in January that he doesn't “believe there will be any difference” between a Trump and a Biden presidency. He argued that Russia-U.S. relations have been going downhill since George W. Bush's administration.

China, where leaders’ initial warmth toward Trump soured into tit-for-tat tariffs and rising tensions, little changed under Biden, who continued his predecessor’s tough stance toward the United States’ strategic rival.

Zhao Minghao, a professor of international relations at Fudan University in Shanghai, said that for China, the two candidates were like “two ‘bowls of poison.’”

Gift, from University College London, said the move to a more fractured world is "going to happen regardless of whether Donald Trump or Joe Biden is elected.”

“It’s just sort of a reality," he said.