Among the notable holdouts are the strongmen who President Donald Trump has cozied up to and heaped praise upon over the last four years. Trump's affinity for authoritarian leaders across the globe has been one of the few constants during his chaotic time in office.

In staying silent, these leaders have spoken volumes about the types of relationships they anticipate having with the new administration.

Russian President Vladimir Putin

Putin wanted Trump to win this election. 

In 2016, the Kremlin congratulated President Donald Trump within hours of the race being called - but Russian President Vladimir Putin has not extended the same message to Biden. On Monday, Kremlin spokesperson Dmitry Peskov said Moscow would wait for official election results before commenting on the outcome.

Over the course of his presidency, Trump broke with longstanding US policy in his repeated praise of Putin, stoking suspicion over his campaign's possible connection to Russian meddling in the election. The same cozy relationship cannot be expected from Biden, who has vowed to treat foreign interference "as an adversarial act."

"Biden will work hard with partners and allies to push back on whatever Russia is up to, whether it's trying to assassinate Russian citizens overseas, or kill their own opposition leaders like the alleged attempt with [Alexey] Navalny in Siberia, or activities in Syria, Crimea, etc.," said Karin von Hippel, director-general of the Royal United Services Institute. "So I do think he [Putin] knows that there will be much more of an effort to try to contain Russia."

Biden will mark a significant step change for Russia, which has had a free hand for some years now - including at the end of President Barack Obama's years in office - von Hippel, a former nonpolitical senior adviser at the State Department under the Obama administration, added.

In late October, Biden called Russia "the main threat" to US national security during an interview with 60 Minutes on CBS. Kremlin spokesman Peskov responded by saying that Russia didn't agree with Biden's remarks, and such rhetoric amplified "hatred towards the Russian Federation."

In the run-up to the election, the two countries did not reach a deal to extend a key arms reduction treaty, New START -- signed by Presidents Obama and Dmitry Medvedev in 2010 -- which the Trump administration was pushing for ahead of election day.

Putin previously indicated that he sees strategic treaties as one of the potential points for cooperation with Biden.

Chinese President Xi Jinping

President Xi is the winner of the US election, and have good reasons not to promote his congratulations. 

Even after his rancorous rhetoric against China as a candidate in the 2016 race, then-President-elect Trump was congratulated on his win by President Xi Jinping, who called for a "sound" and "stable" Sino-US relationship moving forward.

And while Trump and Xi did briefly forge an unlikely friendship over sorbet at the US President's Mar-a-Lago resort, relations between the two countries have deteriorated amid stark divisions over trade, technology, human rights, accusations of Chinese expansionism and - most recently - blame over the Covid-19 pandemic.

But even against this backdrop, Xi has not been quick to welcome a Biden presidency. The Chinese government on Monday sidestepped questions on when it would congratulate Biden on his election victory, with a Foreign Ministry spokesman saying only that China would act in "accordance with international practice."

It's not hard to see why Beijing is hesitant. Biden has boasted of his ability to take on China in contrast to Trump, denouncing the outgoing President for initially embracing Xi. Beijing may not feel obliged to compromise with the US under a new administration, especially as the risk of unpredictable action is considerably lower. But a degree of consistency could also be to Beijing's benefit, von Hippel said.

"Even though Biden will be tough on China, and will work with partners and allies to have a concerted China policy, his platform says we will work with China on areas where there's mutual interest, whether that's climate change or North Korea. And then they'll push back in other areas. So it'll be more nuanced, but I think it'll be better for China because it won't be so erratic and ad hoc like Trump was," said von Hippel.

Turkish President Recep Tayyip Erdogan

Erdogan supported Trump, but have stronger allies with the Democrats sponsors.

As a candidate, Trump praised Turkish President Recep Tayyip Erdogan for his handling of a failed coup attempt despite a major crackdown on perceived opposition figures. As President, Trump trumpeted a controversial referendum win for Erdogan that saw the Turkish leader gain far-reaching and unchecked powers.

In short, with Trump in office, Erdogan has largely been given carte blanche to do what he wants. That will be a very different story with Biden, who Erdogan has not yet acknowledged as President-elect.

Speaking last year on a special episode of The New York Times' "The Weekly," Biden said he was "worried" about Turkey and would be taking "a very different approach" to relations with the country, including supporting opposition leadership and the Kurds.

Trump's retreat from the region - including a sudden withdrawal from Syria that left erstwhile anti-ISIS allies the Syrian Kurds exposed to a Turkish advance - emboldened Erdogan. The Turkish leader has since risked the wrath of the NATO alliance by buying Russian weapons, and backed attacks on US and European interests in the Middle East.

Biden has said Erdogan has "to pay a price" for those actions, including whether the US will continue to sell weapons to him.

Brazilian President Jair Bolsonaro

President Jair Bolsonaro of Brazil, often known as "the Trump of the Tropics" for his shared brand of populist politics, has also kept quiet on Trump's loss.

Bolsonaro and his children -- who, like Trump's, play an active role in politics -- had been hopeful about Trump's re-election bid. His son, Congressman Eduardo Bolsonaro, who wore a "Trump 2020" cap on a trip to Washington as an envoy for his father, questioned Biden's votes and the integrity of the US election on Twitter last week.

Like Trump, Bolsonaro has campaigned and run on polarization, stirring controversy by making misogynistic, racist and homophobic remarks. He has also repeatedly downplayed the Covid-19 pandemic, even as Brazil suffered one of the world's deadliest outbreaks.

With Trump's departure, Bolsonaro loses a diplomatic ally and finds himself facing a US President with a renewed focus on human rights and the environment.

"Will this herald the beginning of the end of other populist leaders? A part of the reason that it may do that is because so many of these populist leaders like Bolsonaro in particular ... are in denial about the pandemic and they really demonstrated to their own people, in many ways as Trump has, that they don't actually care about them," von Hippel said.

Mexican President Andrés Manuel López Obrador

Mexican President Andrés Manuel López Obrador made a carefully worded statement on the US election in which he did not address Biden as the winner and instead said he needed to wait until legal challenges to the vote-counting concluded.

"We are going to wait for all the legal issues to be resolved. We do not want to be reckless. We do not want to act lightly. We want to be respectful of the self-determination of the people and of their rights," López Obrador said on state television Saturday.

López Obrador has forged a close relationship with the US President over the last few years, even in the face of Trump's economic bullying and racist rhetoric. The two men, both populists who've built political brands on a cult of personality, met in July to celebrate the implementation of the United States-Mexico-Canada Agreement trade deal.

López Obrador's reluctance to congratulate Biden may be down to that friendship. The move could also be interpreted as a continuation of a foreign policy tradition of actively avoiding commenting on the affairs of other countries.

The Mexican President added in Saturday's televised statement: "We have a very good relationship with both of the candidates. President Trump has been very respectful with us and we have reached some good accords. We thank him because he has not been a meddler and he has respected us. And the same thing with the candidate Biden. I've known him for more than 10 years."