* Social Democrats lag Merkel ahead of Sept. 22 election
* Steinbrueck was finance minister in 05-09 grand coalition
* Hampered by Merkel's position in political middle ground
BERLIN, June 13 (Reuters) - The centre-left candidate for German chancellor in September's election, Peer Steinbrueck, dismissed Angela Merkel's opinion poll lead on Thursday and said she had lulled the country into a false sense of security.
With 100 days to go until the Sept. 22 election when Merkel will seek a third term, the struggling Social Democrat (SPD) candidate told foreign correspondents the contest would not be decided until the last few weeks of the campaign.
"The election will be decided in the last six weeks, because the German electorate is volatile and many people make up their minds in the last seven to 10 days if and for whom they are going to vote," he said.
"It's true that Frau Merkel's popularity ratings are very high but it's also true that the poll ratings for her government are down in the basement," said Steinbrueck, whose party is some 15 percentage points behind the conservatives in opinion polls.
"Merkel tries to tell the German public 'don't worry, nothing has happened, everything is okay, just go to sleep, I'll take care of it and when you wake up it will all be okay'," he said, raising the pitch of his voice as if to imitate her.
While about six out of 10 people say in polls they would choose Merkel if Germany elected its chancellor directly, rather than via the Bundestag (lower house of parliament), Steinbrueck gets under 20 percent support.
The SPD knows that, to have a chance of unseating Merkel rather than just hoping to be her junior partner as it was in 2005-2009, it must mobilise supporters who did not bother to vote in 2009, when the SPD scored its worst result in post-war history at 23 percent.
"Since our election victory under Gerhard Schroeder in 1998 the SPD managed to lose about eight or nine million voters," Steinbrueck said. Most of those had not defected to other parties, but were in a "waiting room" for non-voters, he said.
Steinbrueck's campaign got off to a bad start. His sardonic humour and high earnings as an after-dinner speaker failed to impress the traditional working-class support base of the SPD and his moderate policies alienated the party's left wing.
Steinbrueck, who served as finance minister in Merkel's 2005-2009 'grand coalition' government with the SPD, finds it difficult to differentiate his policy proposals from hers.
SPD platforms such as a legal minimum wage or protection for tenants have been purloined by Merkel, as has the anti-nuclear power platform of the SPD's Greens allies.
"Merkel continues to define the centre ground broadly and keeps stepping into territory that traditionally belongs to the centre left," wrote Eurasia analyst Famke Krumbmueller.
The SPD's attempts to attack Merkel are hamstrung by its support for her government on the euro zone crisis and by the resilience of the German economy under her leadership, with unemployment at its lowest since reunification.
Apart from complaining that "there are 7.5 million workers in Germany who earn than less than 8 euros ($10.67) a day", Steinbrueck has limited room for manoeuvre beyond promising to put more money into education by taxing the wealthy.
"Ninety-five percent of German citizens won't be affected by our tax proposals," he said, mocking reports by "conservative-oriented business newspapers that the 'cold hand of Socialism' wants to dip its hands into their pockets".
Often acknowledging the respect he feels for Merkel from when they worked together in the financial crisis, Steinbrueck's criticisms tend to be abstract, such as saying Germany has been "governed beneath its value" by the centre right since 2009.