LONDON (AFP) - Barack Obama makes a valedictory visit to Germany on Sunday (April 24) at the invitation of Angela Merkel, a leader who has become his primary European interlocutor and political kindred spirit.

Obama will jet in to Hanover for a final bilateral visit to a country that has long been Europe's biggest economy, but has in modern times punched below its weight politically, diplomatically and militarily.

During Obama's seven years in office, that dynamic has changed, with the US president making the German chancellor, among European leaders at least, first among equals.

Both leaders have an approach to politics that is heavily analytical, leading aides to talk about a relationship that is cerebral and without comparison.

"I consider Angela one of my closest partners and also a friend," Obama told the Bild newspaper, laying on the compliments on the eve of his trip.

"I've worked with her longer and closer than any other world leader, and over the years I've learned from her," he said.

"She embodies many of the leadership qualities I admire most. She's guided by both interests and values." Today, while the United States has a "special relationship" with Britain and France is America's "oldest ally", Germany has become Washington's "indispensable partner".

Obama is ostensibly visiting to attend the Hannover Messe, a trade fair that underscores Germany's commercial prowess.

He will touch down at 12:40pm (1040 GMT), jetting in from London for a two-day visit that kicks off with talks with Merkel, a joint press conference and a trip to the trade fair Sunday.

It will wrap up Monday with a keynote speech and a meeting with Merkel and the leaders of France, Germany and Britain.

For Obama, the trip will be an opportunity to burnish his legacy and politically embrace Merkel, whose fortunes at home have been hit by her handling of the migration crisis.

Critics say her openness to refugees only sped the vast flow of people coming from Syria and beyond.

"I believe that Chancellor Merkel's approach to the refugee crisis - and that of many Germans - has been courageous," Obama said, voicing an opinion heard less often in Germany than Merkel would like.

Despite the diplomatic niceties, the relationship between Obama and Merkel has also been rocky.

They have frequently clashed, most notably over fiscal policy.

Merkel has backed austerity as the remedy to European sovereign debt crises, while Obama came down firmly in favour of short-term spending to buy time and a way out of the morass.

Officials admit US-German relations hit a low when it became known that the US government had been tapping Merkel's phone.

That has helped make the German public among the most sceptical of Obama's leadership in Europe.

According to the Pew Research Center, 45 per cent of Germans have an unfavourable view of the United States.

But officials point to the Ukraine crisis and the downing of flight MH17 as a turning point that helped both leaders begin to work in tandem.

Merkel, according to Obama, "has been essential to maintaining European unity against Russia's aggression against Ukraine." In return, his visit will likely focus heavily on an issue that can help Merkel domestically - trade.

Obama will use the trip as an opportunity to press for a vast US-EU trade deal which the White House still hopes to agree or progress before Obama leaves office in January.

"Germany is not of any military use to the United States: it pays too little into NATO and does too little militarily," said Josef Braml, an analyst at the German Council on Foreign Relations.

"Where they take us seriously is as a European leadership power, and an economic power. And in that sense, we are not just a partner but a competitor. That's why they spied on us." Although Merkel may look to deepen trade to boost job growth, some sectors of the German public remain sceptical.

Tens of thousands of opponents of a proposed trade deal poured onto Hanover's streets Saturday to demonstrate their distaste for the Trans-Atlantic Trade and Investment Partnership (TTIP).

Above all, Obama will use the visit to try to massage his legacy in Europe, giving a speech on Monday that will, for posterity, frame his vision of transatlantic relations.