* German voters against military action in Syria
* Global pressure intense after Libya abstention
* Merkel's party ahead in polls before Sept. 22 vote
By Madeline Chambers
BERLIN, Aug 27 (Reuters) - Just four weeks before she hopes to win a third term as German chancellor, Angela Merkel faces an unexpected problem that could damage her chances - how to respond to shocking pictures of a suspected chemical weapons attack in Syria.
She is caught between overwhelming opposition from German voters to military action in Syria and pressure from international allies to back their calls for "consequences". Berlin's decision two years ago not to support Western intervention in Libya has intensified that pressure.
Merkel, tipped to win a third term in the Sept. 22 vote although she may have to switch coalition partners, knows only too well how international crises can turn elections.
A clear "no" to U.S. plans for an invasion of Iraq in 2002 was widely seen as a crucial factor in handing a surprise victory to Social Democrat (SPD) Gerhard Schroeder.
Few experts think Merkel will depart radically from the traditionally cautious German stance on military action abroad - a pacifist strain that derives from the collective shame of having started World War Two and perpetrated the Holocaust.
A poll on Tuesday showed 69 percent of Germans were against a military strike in Syria and only 23 percent backed it.
However, the conservative Merkel and Foreign Minister Guido Westerwelle, a member of her Free Democrat (FDP) junior partners, ratcheted up the rhetoric this week.
Merkel's spokesman described the suspected use of nerve gas by government forces in Syria's civil war as "breaking a taboo", adding that a very clear international response was needed. He declined to speculate on possible military intervention.
The United States and its close NATO allies are drafting plans for air strikes and other action after the reported chemical attack which killed hundreds of Syrian civilians.
Western powers could attack Syria within days, envoys from the United States and its allies have told rebels fighting President Bashar al-Assad, sources who attended the meeting told Reuters on Tuesday.
None of this means Germany will actually offer more than moral support and some logistical help, but it highlights a recognition that Berlin will be marginalised if it decides against standing by its allies, as it did in 2011 over Libya.
The top-selling Bild daily ran a story on Tuesday with the headline "Can we really keep out?" and offered the answer that it would be "difficult". It quoted a lawmaker saying Germany could not afford to isolate itself again after the Libya vote.
In 2011 Germany abstained in a U.N. Security Council vote on action against Libya, angering Washington and other allies.
Karl-Georg Wellmann, a member of Merkel's Christian Democrats (CDU), spoke out for Western intervention in Syria.
"We can't look on passively at mass murder," he told Spiegel Online, saying a military signal was needed and that Berlin may give logistical or missile defence support.
Earlier this year, Germany sent Patriot missiles and about 300 soldiers to Turkey as part of a NATO mission to help protect its easternmost member state from war spillover from Syria.
In a sign that Germany wants to be involved in decisions now, its military chief of staff attended weekend talks in Jordan with alliance counterparts on the Syria crisis.
But pollsters say Merkel faces a delicate balancing act.
"After Libya, the question of moral support and cooperating with the United States and its allies is more urgent," Emnid pollster Klaus-Peter Schoeppner told Reuters.
"But the government won't risk alienating the big majority against military action," he said.
The opposition SPD, trailing Merkel's conservatives by at least 15 points, hope to win votes by stressing the risks.
Their challenger to Merkel, Peer Steinbrueck, warned against behaving as if military action could end civil war and promote peace in Syria. SPD lawmaker Gernot Erler went even further, saying: "We must do everything to avoid a military escalation." (Editing by Stephen Brown and Mark Heinrich)