US President Barack Obama believes Chancellor Angela Merkel is certain to be re-elected when Germany goes to the polls on September 27, according to the news magazine Der Spiegel. "You've already won," the president told the chancellor when the two met during Merkel's visit to Washington on June 26, the magazine said in a report to be published Monday, according to dpa. Germany's semi-public television channel ZDF picked up the comments and broadcast them at the time, but they went unreported by other media. At a press conference, Obama praised the German leader. "Chancellor Merkel is smart, practical, and I trust her when she says something," he said. The remarks apparently piqued German Foreign Minister Frank-Walter Steinmeier, the Social Democrat who is challenging the conservative Merkel for the chancellorship. Der Spiegel quoted an aide to Steinmeier as saying the foreign minister "thinks a great deal of Obama, but even an American president is not a prophet." The German electorate will decide who the next chancellor is, the magazine quoted the aide as saying. Current opinion polls show Merkel's Christian Democrats (CDU) around 15 per cent ahead of Steinmeier's party. The chancellor hopes to poll enough votes to form a coalition with the pro-business Free Democrats and jettison the Social Democrats, with whom she has ruled in a grand coalition for the past four years.