"I never thought I would have to say something like this," Merz said in a TV debate late Sunday before calling for an "independent European defence capability" as a possible alternative to "NATO in its current form".

As Germany's likely next chancellor, Merz lost no time in addressing the challenge posed by US President Donald Trump and his series of unprecedented threats that have unsettled NATO allies.

Most dramatically, Trump has reached out to long-isolated Russian President Vladimir Putin, over the heads of Kyiv and Europe, in a bid to end the war and labelled Ukraine's President Volodymyr Zelensky a "dictator".

The new US administration has also stepped up calls on NATO allies to sharply raise defence spending and secure a future Ukraine ceasefire alone, as America looks to shift its strategic gaze to Asia.

Merz in a post-election press conference Monday said he hoped that Europeans "can convince the Americans that it is in our mutual interest to maintain good transatlantic relations".

"But as always, you have to deal with the worst-case scenario," he said.

"All the signals we are receiving from the United States indicate that interest in Europe is decreasing."

Until recently, such a policy shift would have been unthinkable in Germany, which after World War II built its democracy under American tutelage and grew prosperous under the US-led security umbrella.

Germany hosts about 35,000 US forces on multiple bases, the largest contingent in Europe, and a legacy of the Cold war.

For many years, Berlin has resisted calls from Paris to build up Europe's defence capabilities, feeling secure under the protection of the nuclear-armed superpower America.

- Nuclear questions -

On Sunday, a victorious Merz quickly spoke with Emmanuel Macron as the French president headed to Washington to meet Trump, who on Thursday also holds talks with British Prime Minister Keir Starmer.

Merz has for months signalled the need to make Europe "more sovereign and to free it from historical dependence on the USA", said Jacob Ross of the German Council on Foreign Relations.

But Ross added that Merz's statement on Sunday was "surprising in its severity".

"If he actually acts in accordance with this statement in coming weeks, it would amount to a break with the transatlantic relationship as we have known it in Germany so far."

However, Ross stressed that for the committed transatlanticist Merz, "it is not an active break, but rather a reaction to the political developments in the USA".

Merz, speaking Friday to public broadcaster ZDF, had already argued that Europe must now "make the greatest efforts to at least be able to defend the European continent on its own".

This would include talking to Britain and France, which both have atomic weapons, about a nuclear shield for Europe, he said.

The idea of joint "nuclear independence" in Europe had been discussed for years among foreign and security policymakers, Merz said, expressing regret that no solution had yet been found.

Previous German governments of all stripes have resisted a push from France to develop Europe's strategic nuclear autonomy away from the United States.

- 'Sovereign Europe' -

Ross said that now Macron, "shortly before the end of his second term, has a rather unexpected opportunity to bring his vision of a sovereign Europe to life".

"One could even say that this is a historic opportunity for French politics after decades of promoting European sovereignty and Gaullist-inspired ideas.

"The fact that Britain could be part of this development would be remarkable both against the backdrop of Brexit and the 'special relationship' between the the United States and Britain."

Whatever happens in future, Europe would face momentous challenges if it needs to replace departing US forces, said Liviu Horovitz of the German Institute for International and Security Affairs.

Berlin has only just managed to meet NATO's target of spending two percent of GDP on defence, and Trump now demands allies raise the target to five percent.

For a Merz-led government yet to take shape to pour in the enormous funds needed, it would have to ease the so-called debt brake enshrined in Germany's constitution.

Here a challenge looms as Merz would likely meet opposition from two parties that will hold a blocking minority in the next parliament -- the far-right AfD and the far-left Die Linke.

If Merz indeed wanted to relax the debt brake, he would need votes from either of those parties, said Andreas Bock of the European Council on Foreign Relations.

"Although Die Linke is fundamentally opposed to the debt brake, it is unlikely to be willing to soften it for the sake of higher defence spending," Bock said.