Berlin  - Germany's Social Democratic Party (SPD) is to put up a candidate to challenge incumbent President Horst Koehler for office next May, SPD officials announced in Berlin Monday.

Gesine Schwan, 65, a professor of politics who narrowly lost the May 2004 election to Koehler, is to stand again, the SPD, the junior partner in Chancellor Angela Merkel's broad coalition, said.

Schwan is president of the Viadrina European University in Frankfurt an der Oder on Germany's border with Poland.

The announcement, which had been widely anticipated, has been strongly criticized by Merkel's Christian Democratic Union (CDU) and its Bavarian sister-party, the Christian Social Union (CSU).

Merkel's conservative Christian bloc has expressed anger that Schwan will be relying on the backing of the socialist Left Party, whose main support base is among former communists in the east of the country.

However, Merkel's official spokesman Monday rejected speculation that the coalition, which draws together the two main rival blocs of postwar German politics, would break up ahead of the September 2009 federal elections.

"I assume the work of the government will proceed... I don't believe anyone in the three parties doubts the ability of the coalition to continue its work over the next 16 months," Ulrich Wilhelm.

On Thursday last week, Koehler, a CDU member, announced he would seek election next year to the largely ceremonial post for a second and final five-year term.

Merkel welcomed the announcement. "Horst Koehler has reached into the hearts of the people," she said.

Koehler, 65, is Germany's ninth postwar president. He succeeded Johannes Rau in 2004.

He was managing director of the International Monetary Fund (IMF) before becoming German president and previously headed the European Bank for Reconstruction and Development.

Although Koehler enjoys considerable popularity among ordinary Germans, observers said that the outcome of the election was uncertain.

The German president is elected every five years by a Federal Assembly that sits purely for this purpose and comprises members of the lower house of parliament, the Bundestag, plus an equal number of representatives drawn from the 16 states.

Currently the parties backing Koehler - the CDU, the CSU and the liberal FDP - would have a majority in the assembly.

But Bavarian state elections in September could shift the balance towards the SPD, whose candidate would hope to receive the backing of the Greens and the socialist Left Party, although both parties have thus far kept their position open.

The far-right NPD has said it aims to put up its own candidate in cooperation with another far-right grouping, the DVU. Together the two parties would have possibly four votes of the more than 1,200 in the assembly, but these could prove critical in the event of a close outcome.

The election, by secret ballot and without debate, is to be held on May 23 next year. A maximum of two terms in office is permitted. (dpa)