* Steinbrueck fights on but lags far behind Merkel in polls
* Uncertainty grows as SPD cools on "grand coalition" option
* SPD candidate shows human side in a bid to rescue campaign
By Erik Kirschbaum
HAMBURG, Germany, Aug 15 (Reuters) - Peer Steinbrueck, fighting a long-shot battle to oust Chancellor Angela Merkel in September, was determined to get into the cockpit of an empty Lufthansa Boeing 747 so that a pack of photographers could get shots of him at the controls.
But he forgot the 747 cockpit is on the upper deck and ended up stuck in front of a windowless wall on the floor below with a puzzled look on his face.
Steinbrueck, a straight-talking former finance minister, can't seem to convince voters that he would be a better pilot than Merkel. Germans are in no mood for change, polls show, and Steinbrueck is struggling to get his campaign off the ground.
In fact, with less than six weeks to go until the Sept. 22 vote, the debate within his party, the centre-left Social Democrats (SPD), is shifting.
No longer are SPD members asking whether Steinbrueck can win. They are trying to figure out what to do when he doesn't.
A growing number of voices in the SPD is warning the leadership of Germany's oldest party against entering another "grand coalition" under Merkel - which could lead to a period of uncertainty as she searches for another coalition partner.
The last time the SPD joined forces with Merkel, the party crashed to its worst result in the post-war era, winning just 23 percent of the vote in 2009, down from over 34 percent in 2005.
Several SPD leaders suggested recently that the SPD might be better off on the opposition benches if the party and its traditional partner, the environmentalist Greens, falls short of a majority in September.
"If that's the result we end up with, then it is likely the view will prevail that the SPD should not go into a grand coalition," former party chairman Franz Muentefering told German weekly Die Zeit in an interview published on Tuesday.
Some are also interpreting plans by the SPD, announced by chairman Sigmar Gabriel this week, to hold a special congress two days after the vote as a sign the party could refuse a partnership with Merkel, who may be forced to seek a new coalition partner if she fails to win a centre-right majority.
Steinbrueck has ruled out participating in another "grand coalition" under Merkel. His ally Frank-Walter Steinmeier, the SPD parliamentary leader and a former foreign minister who was crushed by Merkel in 2009, is also known to be deeply sceptical.
Perhaps more importantly, so is Hannelore Kraft, the popular SPD state premier of North Rhine-Westphalia, whose influence could grow significantly if the party turns in another disastrous performance next month.
"It's going to be a very different situation than in 2005," said Thomas Jaeger, a political analyst at Cologne University. "I can imagine there are a lot of people in the SPD who aren't going to be interested at all in being junior partners with Merkel again. They saw where that led them last time."
Steinbrueck is trying desperately to narrow the gap with Merkel's conservatives.
He has abandoned the traditional German stump speech in front of large crowds. Instead, he is opting for more U.S.-style town hall events in which he stands in the centre of a smaller group, fielding questions and doing his best to entertain.
"It's a different kind of event," Steinbrueck told a small group of reporters. "We saw with Obama how his campaigns went straight to the streets to give voters a direct comparison."
While it worked for Obama, Steinbrueck has so far failed to capture Germans' imaginations. In fact his campaign has been filled with blunders.
He mishandled a row over lucrative fees he earned as an after-dinner speaker before becoming the party's candidate. He then compounded the error by saying chancellors were underpaid and that Merkel was so popular because she was a woman.
Last week, thousands of campaign posters dissolved in the rain.
Opinion polls have put the SPD at around 25 percent for months - roughly 15 points behind Merkel's conservatives.
A Forsa poll published on Wednesday showed only 23 percent of Germans would vote for Steinbrueck if they could directly elect their chancellor, up 2 points from last week. Merkel would win 54 percent of the direct vote, down 1 point.
The poll contained more bleak news for Steinbrueck as it found nearly a quarter of SPD voters believe Merkel and her conservatives are better able to tackle Germany's problems.
Support for the Greens, junior partners to the SPD in former Chancellor Gerhard Schroeder's government between 1998 and 2005, stands at roughly 13 percent.
The Greens and the conservatives were once enemies but have grown closer. Some analysts said a conservative-Greens alliance would be an option for Merkel if she failed to get a centre-right majority and the SPD balked at a coalition.
In one of Steinbrueck's favourite campaign jibes at Merkel, he describes her as a pilot who has lulled the passengers into a false sense of security by guiding them through the storm.
"The problem is," Steinbrueck says, pausing for his punch line: "You just don't know where you're going to land."