By Erik Kirschbaum
BERLIN (Reuters) - Chancellor Angela Merkel warned her supporters on Saturday against complacency ahead of the September 22 election, voicing fears that three leftist parties could unexpectedly join forces to oust her center-right coalition after ballots are counted.
Even though Merkel's conservatives hold a 16-point lead over the Social Democrats (SPD) in opinion polls, the chancellor told a rally in Cloppenburg the SPD might still break a vow and form a coalition with the ostracized far left "Linke", or Left Party.
The SPD and Greens, who have ruled out a coalition with the Left, trail Merkel's center-right coalition by about eight points in polls. But the ruling coalition fears the SPD and Greens will ignore past promises and form a government with the Left, popular in the east and on 8 percent in voter surveys.
"Those who think the election is already in the bag and Merkel will still be chancellor no matter what might be in for a rude awakening after the polls close," Merkel said, speaking in Lower Saxony state where her Christian Democrats lost power to the SPD and Greens in a stunning upset in January.
"Those who think this election is already won might wake up and see Germany has a government with the Left Party in it," she added, articulating the governing coalition's fear that the lure of power could be too strong for the SDP.
The Left traces its origins to the Communist Party in the former East Germany and has been ostracized by other mainstream parties.
"The election outcome is anything but certain," said Merkel, who saw big opinion poll leads before the 2005 and 2009 elections melt away. She launched her 56-stop election campaign on Wednesday.
An opinion poll published on Friday showed Merkel's center-right coalition was in a 46-46 dead heat against three leftist opposition parties. If her center-right coalition falls short, many analysts expect the conservatives and SPD to form another "grand coalition".
But SPD chancellor candidate Peer Steinbrueck, who spoke at a rally to some 300,000 in Berlin on Saturday, rules that option out for himself. Many in the SPD oppose teaming up with Merkel as the SPD's support crumbled after the last grand coalition.
"I've never seen a sight like this," said Steinbrueck, at the rally at the Brandenburg Gate, part of celebrations this year marking the SPD's 150th anniversary.
He drew applause during his 50-minute speech for promising to introduce a minimum wage should he take office.
"We need more 'we together' in this country again and not millions of sharp elbows," he said. "I want Germany to be a country where it doesn't matter where you come from but where the only thing that matters is where you want to go."
(Additional reporting by Andreas Rinke in Cloppenburg; Editing by Jon Boyle)