* Steinbrueck chokes up after wife tells how lives changed
* Social Democrats badly lagging Merkel in polls
* Steinbrueck rebukes SPD chairman (Recasts with Steinbrueck choking up at event)
Peer Steinbrueck, the main opposition challenger to German Chancellor Angela Merkel, choked up during an interview on Sunday when he was asked about what motivated him to carry on despite all the pressure and criticism of his campaign.
In an extraordinary display of emotion from a man known for his abrasive style, Steinbrueck struggled to find words, and sighed heavily. As his eyes welled up, the audience at the event by his Social Democrat (SPD) party stood up and applauded.
Steinbrueck, who is badly lagging Merkel in the run-up to the Sept. 22 election, faced more controversy on Sunday after rebuking his party's chairman for his disloyalty - yet another sign of disarray in his campaign.
He gave an interview with his wife Gertrude at the party event in which she spoke about how their lives had totally changed since her husband became lead challenger, and said he would never have taken this on, were it not for his convictions.
"We were doing well ... we had freedom, we had free time, we could decide what we wanted to do," she told the interviewer at the televised event. "Then he became challenger, and gets nothing but criticism for everything he has done before."
When the presenter then turned to Steinbrueck to ask what his motivations were, his eyes welled up.
The SPD trail Merkel's conservatives in opinion polls by about 15-16 points. In an Emnid poll published on Sunday, they slipped a point to 25 percent, their lowest since January, and only 2 points above their poor showing in the 2009 election.
The SPD had high hopes of beating Merkel when it chose Steinbrueck last September. He was seen as an acerbic, conservative voice in the left-leaning SPD who could woo less convinced voters away from Merkel.
But his message has been clouded by mistakes. He became mired in a row over earning 1.25 million euros ($1.7 million) as an after-dinner speaker, and bungled an ensuing public debate.
Six days ago, he sacked his chief spokesman, widely blamed for failing to prevent damaging remarks he made - that German chancellors were underpaid and Merkel was so popular because she was a woman - from appearing in a newspaper in December.
Deepening the sense of a lack of unity, Steinbrueck, 66, told weekly Der Spiegel he expected all SPD members, including chairman Sigmar Gabriel, to back him, following a meeting in which Gabriel openly criticised him.
"Only by concentrating all our strengths can the SPD oust this government and Mrs Merkel," the former finance minister told Der Spiegel.
"I therefore expect that everyone, including the party chairman, will over the next 100 days get behind the chancellor candidate and the campaign in a loyal, constructive manner."
At the party event in Berlin on Sunday the men sat next to each other and denied any rift. Gabriel said there could sometimes be friction, much as in a marriage, but this ultimately only generated warmth.
Despite her strong lead, Merkel may still struggle to form a government if her allies, the liberal Free Democrats (FDP), fail to secure the 5 percent minimum for entry into parliament. She might then be forced to consider a coalition with the SPD, like the one she led in 2005-9.
FDP lead candidate Rainer Bruederle broke his hand and his leg in an accident on Friday, a party spokesman said on Sunday, adding he expected him to be able to return fully to work in two weeks' time.