By Madeline Chambers
BERLIN (Reuters) - Still reeling from a state election rout which unleashed a party row about her open-door migrant policy, Angela Merkel's conservatives are bracing for further losses in the Berlin city vote on Sunday.
The chancellor's decision a year ago to open German borders has hit her popularity and dominated the campaign, boosting support for the anti-immigrant Alternative for Germany (AfD)party.
Polls show the center-left Social Democrats (SPD) may be able to drop Merkel's Christian Democrats (CDU) as coalition partners in the capital's assembly.
Merkel, cheered by 2,500 conservative supporters at a recent rally in the leafy western suburb of Lichterfelde on a sunny evening, knows what is at stake, especially as it is only a year unril the next federal election.
Since the CDU was pushed into third 10 days ago by the AfD in the eastern state Mecklenburg-Vorpommern, her conservative CSU allies in Bavaria have blamed Merkel personally and demanded a migrant cap, which she rejects.
And while she has no obvious rival, the losses have raised questions about whether she will even run for a fourth term in 2017.
Merkel defended her policy, appealing to Berlin's openness.
"Berlin, its whole history, the success of what was West Berlin, its openness has served it well and must be preserved," she said, stressing the humanitarian duty to help war refugees.
Since the fall of the Wall, 27 years ago, Berlin has transformed itself from the front line of the Cold War into a trendy capital, attracting artists and start-up entrepreneurs although it accounts for only 4 percent of the German economy.
Many CDU voters say they are worried about the crisis which saw about 1 million refugees enter Germany last year. Some 80,000 arrived in Berlin, a city of 3.5 million. Voters are focused on the cost, integration and security.
"Merkel made a mistake letting everyone in. She will pay the price and so will Germany, our children," said Moritz Daul, 48, who will nonetheless vote CDU. He said Merkel's days were numbered but she was the best chancellor candidate for now.
Merkel's problem was very evident at the rally on Wednesday.
Up to 30 hecklers booed, whistled and yelled "Merkel must go". One, sporting a German eagle on his T-Shirt and the slogan 'Wir sind das Volk' ('We are the People'), said he would vote AfD.
The slogan was coined by East Germans protesters before the end of Communism and has since been adopted by the anti-Muslim PEGIDA group,
The AfD, which has won seats in nine of Germany's 16 states, has successfully played on immigration fears. Berlin candidate Georg Pazderski has said: "I favor educating these people (immigrants) but not integrating them. We must prepare them for going back."
An INSA poll this week put the CDU on 18 percent in Berlin, down more than five points from the 2011 vote and only four points ahead of the AfD.
The SPD - which is in coalition with Merkel at the federal level - is expected to remain the biggest party in Berlin and aims to form a coalition with the Greens and radical Left. They are led by printer Michael Mueller, who acknowledges he falls short in the "glamour" stakes compared with his party-loving predecessor Klaus Wowereit who dubbed Berlin "poor but sexy".
Carsten Koschmieder, political scientist at Berlin's Free University, predicted further damaging splits between the CDU and CSU if voters reject conservatives in Berlin.
"Critics of Merkel will get louder while her supporters in the CDU will blame (CSU leader) Horst Seehofer for using destructive rhetoric," he said.
(Editing by Jeremy Gaunt)