The election saw the AfD win seats in parliament for the first time - a political natural disaster that followed Merkel's 2015 decision to leave open German borders to more than 1 million migrants.
Almost five months after the national election, Germany is still without a federal government as the SPD consults its members before embarking on a re-run of their "grand coalition" with Merkel. The SPD's youth wing has been energetically campaigning against the coalition, arguing that another power-sharing deal with the Christian Democrats will be a death knell for the party. The SPD lost one more percent of support and can count on only 15.5 percent of votes.
AfD parliamentary leader Alice Weidel tweeted that the group had officially become a people's party.
Bild called the latest survey "a bitter blow" for the venerable SPD.
Some 463,723 registered members of the SPD are eligible to vote on accepting or rejecting a revival of the centre-left and centre-right partnership that has governed Germany since 2013.
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Poggenburg made the derogatory comments during a party rally in Nentmannsdorf near Pirna, eastern Germany, where he also called immigrants with dual passports as "homeless mob that we no longer want to have here". After the voting, the coalition talks began formally, however, it has failed so far to result in a stable government. In the event of a no vote, expect intense lobbying from CDU senior figures for Merkel to form a minority government.
The Social Democratic Party's 464,000 members will vote in a postal ballot today on whether their party should go ahead with the coalition agreement its leaders clinched this month to renew their alliance with German Chancellor Angela Merkel's Christian Democratic Union (CDU) and Christian Social Union (CSU) bloc.
The shift, Der Spiegel judged, is in line with "the fragmentation of society, greater individualism, the dissolution of traditional socio-economic groups.and the logic of social media".
The Insa poll, carried out for the top-selling Bild newspaper, put the AfD ahead of the crisis-wracked Social Democratic Party for the first time.
"That is why the AfD will shrink once a new government is in place", he predicted.