By Erik Kirschbaum
BERLIN (Reuters) - German President Horst Koehler faces a tricky battle for a second five-year term on Saturday that will serve as a test for Chancellor Angela Merkel's conservatives ahead of a parliamentary election in September.
Koehler, a former head of the International Monetary Fund who has held the ceremonial post since 2004, should have just enough support from Merkel's conservatives and their allies to secure victory in the 1,224-seat Federal Assembly.
A surprise win for his challenger Gesine Schwan, a feisty university president supported by the Social Democrats (SPD), would severely embarrass Merkel just four months before a federal parliamentary election in which she is seeking a second four-year term.
Schwan lost to Koehler by 604 votes to 589 five years ago despite siphoning some support from conservative ranks. She hopes to go one better this time in what could be a close race.
With backing from Merkel's conservative CDU, its CSU sister party, the liberal Free Democrats and the small Bavarian Free Voters party, Koehler on paper has 614 votes -- just one more than required for an absolute majority in the assembly.
The parties backing Schwan -- the SPD, which shares power with Merkel in a loveless grand coalition, and the Greens -- have 514 seats. But the opposition Left party is likely to throw its 90 votes behind her after the likely elimination of its candidate, actor Peter Sodann, in round one of voting.
"Every election result this year before the parliamentary election is being used by the parties to mobilise supporters and this presidential election is vitally important," said Gero Neugebauer, political scientist at Berlin's Free University.
"The conservatives need a win to show they're disciplined and to lay the groundwork for a CDU-FDP government they both want to have in September."
UNPREDICTABLE DELEGATES
Half of the assembly is drawn from the 612 members of parliament and half from delegates sent by the country's 16 states. These are often local celebrities or sporting figures who are not always firmly aligned to parties, creating an element of unpredictability.
Juergen Falter, a political scientist at Mainz University, said a surprise victory for Schwan could not be ruled out.
"It would give the SPD an enormous boost if Schwan were to win," he said. "It would be a major defeat for the CDU-FDP hopes in the parliamentary election later this year."
By tradition there is no campaigning for the office.
But Schwan spent the last year trying to woo support from the CDU and Left after she won over some defectors in 2004.
Koehler has also broken with protocol and criticised Schwan for saying she feared the economic crisis was creating an explosive social atmosphere in Germany.
Ahead of the September 27 federal vote, Merkel's CDU leads in opinion polls but is unsure of holding the chancellery due to complex coalition arithmetic.
(Editing by Mark Trevelyan)