A return of sorts for the former US president - in Germany

Campaign posters featuring Barack Obama have gone up in Berlin. The posters highlight Obama's support for German Chancellor Angela Merkel. Photo / The Washington Post

Barack Obama was on his final overseas trip as US President last November, with the wounds of Donald Trump's victory still fresh, when he ad-libbed an unusual endorsement for his host, German Chancellor Angela Merkel.

"If I were German and I had a vote, I'd support her," Obama said as Merkel stood by his side at a news conference in Berlin. "I don't know if that helps or hurts."

Nearly a year later, Merkel's allies clearly believe it helps.

The evidence has come in recent days at Berlin bus stops and subway stations, parks and central squares. Obama - who hasn't run for office in five years - is back on campaign posters, promoting Merkel's re-election bid.

The image is familiar: the stylised stencil of Obama's million-mile gaze, as depicted by artist Shepard Fairey in his iconic 2008 "Hope" poster.

But this time, the poster's red, white and blue has been juxtaposed with Germany's red, black and gold, along with the logo for Merkel's party, the Christian Democratic Union (CDU).

Beneath Obama's face is a translation of last year's endorsement: "Wenn ich könnte, würde ich Merkel wählen".

The decision to feature a former US president in a German election campaign was made by a Merkel ally who is running for Parliament in Berlin's leafy southwest. It's a highly unusual move, but one that reflects Obama's special status here.

He transfixed Germans with his speech beside Berlin's Victory Column during the 2008 campaign, and he later bonded with Merkel over the details of issues such as climate change and Russian sanctions.

When Obama left office, 86 per cent of Germans said they trusted him to do the right thing in global affairs, according to the Pew Research Centre. His successor, Trump, earns the trust of only 11 per cent.

The reminder that Obama gave Merkel such enthusiastic support might help to win over wavering voters - especially the young, said CDU activist Tom Cywinski.

"Obama moved people to become engaged in politics with his slogan 'Yes we can,' " said Cywinski, 29, who was among those who came up with the idea to feature the former US President. "He's the right face, especially in times of populism."

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The posters are also a subtle rebuke of Trump, said Thomas Heilmann, the CDU candidate whose campaign has put up about 100 of the posters in recent days.

"This is a nice way of saying, 'We prefer the former one,' " Heilmann said. "This is how we'd like to see America as a nation, compared to what Trump is doing."

Heilmann, a 53-year-old entrepreneur who is seeking to unseat an incumbent from the centre-left Social Democratic Party, said he didn't consult with either Merkel or the Obama Foundation before producing the posters.

But he said the feedback he has received from supporters has been uniformly positive, even though his party is technically right of centre. "Ninety-five per cent of all people in the CDU would be Democrats in the US," he said.

Merkel doesn't necessarily need Obama's help.

A plurality of German voters already seem to agree with the former US President that Merkel deserves re-election. With less than two weeks to go before the September 24 election, polls show the CDU on track for a comfortable victory - and Merkel headed for a fourth term.