In the twilight of his political career, Joe Biden is disappearing into the background even before Donald Trump returns to the White House, despite growing turmoil at home and abroad.

The 82-year-old US president's silence on a looming government shutdown over Christmas is just the latest example of how critics say the ageing leader is in office but not in power.

Despite still occupying the world's most powerful pulpit, Biden has remained virtually absent from the public debate about his noisy successor whom he often used to brand as a threat to democracy.

While Biden stays largely off the radar, it is Trump who increasingly calls the shots, whether by carrying out his own shadow foreign policy or by torpedoing a deal to fund the government.

"This is a Biden problem to solve!" Trump said on his TruthSocial network Friday.

Biden did make a public appearance on Friday at the Children's National Hospital in Washington but did not address the shutdown threat -- instead playing a game of catch with an energetic toddler.

Under intense questioning on Friday, the White House defended Biden's silence on the stand-off with Republicans.

"They blew it up, and they need to fix it. This is not for the president to fix," Press Secretary Karine Jean-Pierre told reporters.

It was always going to be a painful exit for Biden, after he was forced to drop out of the 2024 election in July following a disastrous debate against Trump.

He handed over the Democratic nomination to Vice President Kamala Harris yet was widely reported to have believed that he could have succeeded where she failed against the Republican.

The White House insists that Biden is in what his chief of staff Jeff Zients called a "sprint to the finish line" to preserve his legacy before the January 20 handover of power.

In recent weeks he has unveiled a string of last-gasp efforts to "Trump-proof" policies on everything from student loan forgiveness to the economy.

He gave televised speeches to hail a ceasefire in Lebanon and to claim credit for the fall of Syrian President Bashar al-Assad -- while his administration is still pushing for a truce in Gaza.

Yet sometimes his efforts have often underscored how America's oldest president is fading away more than going out with a bang.

Biden coughed throughout what was meant to be a showpiece speech to trumpet his economic legacy last week.

He also appeared frail in a trip to the Amazon during which he momentarily looked like he was wandering off into the jungle, and missed a group photo with G20 leaders in Brazil when they refused to wait for him.

In Angola, making a long-promised first trip of his president to sub-Saharan Africa, Biden appeared to be resting his eyes for several periods during a summit

Moreover, the man who was once one of the most garrulous politicians in Washington has been almost silent in the face of reporters' questions.

Biden has in particular declined to say whether he still believes Trump remains a threat to democracy, despite repeatedly condemning the Republican during the campaign for his actions during the January 6, 2021 attack on the US Capitol.

He has also refused to comment on perhaps his most prominent act since the election defeat, a pardon for his troubled son Hunter on gun and tax charges.

The sense of a Biden fade-out has not been helped by a proliferation of stories in US media about his decline.

The Wall Street Journal reported this week that White House gatekeepers built a wall around a "diminished" Biden during his presidency, keeping meetings short and controlling access.

The White House denied that Biden had declined physically or mentally, it said.

David Karol, who teaches government and politics at the University of Maryland, said that on the shutdown Democrats had "made the political calculation that it is best to leave the spotlight to Trump and (close ally) Elon Musk."

Trump "really never went away. By contrast, Biden has been the quietest president in decades," Karol told AFP.