"The aphorism 'becoming chancellor isn't difficult, but being chancellor is' applies to some politicians," said former Social Democratic Party cabinet minister Erhard Eppler. "But in the case of Peer Steinbruck, it seems to be the other way around."
Steinbruck, Merkel's main challenger in Germany's federal elections, has not run a smooth campaign. The centre-left SPD's chancellor candidate does not shy away from saying what's on his mind. This has led to some awkwardness: for instance, in August he said Angela Merkel "lacked passion" for Europe due to her upbringing in the former East Germany - comments that were pilloried by the German media.
Born in 1947 in Hamburg, Steinbruck studied economics at the University of Kiel, and in the 1990s served as minister of economics in the states of Schleswig-Holstein and North Rhein-Westphalia. In 2002 he became "minister president", or governor, of North Rhein-Westphalia, Germany's most populous state.
After federal elections in 2005, Germany's two biggest parties - the SPD and the centre-right CDU - formed a "grand coalition", with the CDU's Merkel as chancellor. Steinbruck was named Germany's finance minister, and was serving in this position when the global financial crisis began in 2008. Germany avoided much of the fallout, thanks in part to a highly regulated banking sector, and unemployment has steadily declined since 2009.
In Germany's 2009 elections, the CDU and the liberal FDP formed a ruling coalition, and Steinbruck - his party out of government - returned to being a member of parliament.
During this time, from 2009-12, he made 1.25m euros ($1.65m) giving paid lectures. This, in addition to an awkward statement in which he argued that the salary paid to chancellors was too low, have led some to perceive Steinbruck as mercenary.
The SPD's chancellor candidate has also clashed with Sigmar Gabriel, the party leader, whom Steinbruck said had not been supportive of his campaign.
In federal elections, Germans vote for their party of choice, and not directly for their preferred chancellor. This is good for Steinbruck: Polls show that if Germans could pick between Merkel and the SPD chancellor candidate, they'd pick Merkel by a wide margin, with only about 30 percent of voters saying they prefer Steinbruck. Nevertheless, unless Germany's three left-wing parties agree to band together in a coalition, he does not look likely to oust Merkel as chancellor.
In the run-up to the elections on September 22, Steinbruck has emphasised the need to reduce economic inequality and implement a national minimum wage, criticised Merkel's support for hardline austerity measures in indebted eurozone countries, and lambasted the Merkel-led government for failing to shield Germans from widespread US electronic surveillance.
Steinbruck's wife, Gertrud, is a former biology and politics teacher at a high school in Bonn. They have three children.