Merkel, who has led Germany for 11 years, last month confirmed she would run for a fourth term but acknowledged that the election would be "more difficult" than any other she has contested.
Party faithful from her centre-right Christian Democratic Union (CDU) gathering for a two-day annual congress in the western city of Essen are expected to overwhelmingly re-elect Merkel as party chief, rallying behind her bid to stay in power.
"I'm counting on an honest result," she told public broadcaster ARD, as national media suggested that any score below 90 per cent would be a slap in the face.
Crucially, she will also be grilled on how she plans to stop the populist anti-Islam AfD from further eroding the party's supporter base.
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The CDU has suffered setbacks in five consecutive state polls as voters punish Merkel for her liberal refugee policy, with more than a million people seeking asylum in Germany last year.
There is concern within CDU rank and file, because Merkel has said she "will stand again, without saying how she will change her policies in the future", Hans Pistner from the Thuringia regional government told regional broadcaster MDR.
Merkel's CDU and its Bavarian sister party CSU secured a decisive win of 41.5 per cent at the last election in 2013 -- its best result since national reunification in 1990, on the back of strong approval for her tough stance on austerity for debt-stricken EU nations.