The Alternative for Germany's telegenic co-leader Frauke Petry had already announced last week she would not join the campaign squad, after weeks of bitter infighting between populists and more radical, hard-right forces.
Petry, a 41-year-old former chemist pregnant with her fifth child, was handed a further setback Saturday at the gathering in the western city of Cologne, which drew tens of thousands of protesters.
The around 600 delegates rejected her call to adopt a more moderate-sounding "Realpolitik" programme intended to shut down the party's more extremist voices, including those who have attacked Germany's Holocaust remembrance culture.
Top-selling daily Bild called delegates' decision to not even debate her motion a "crushing blow" for Petry, who expressed bitterness on the sidelines of the meeting.
"I will step aside during the campaign, as that's what the party congress apparently wants," Petry said, while pledging to remain party co-chairwoman "for now".
"As long as the party is not willing to say in what direction it wants to go, a team will have to lead the campaign that can deal with this indecision better than I can."
The AfD has seen its support plummet as the refugee influx to Germany has slowed in recent months after Chancellor Angela Merkel let in more than one million asylum seekers since 2015.
The party, now represented in 11 of Germany's 16 states, aims to sign off on a programme that will pave the way for it to enter the national parliament for the first time in its four-year history.
It includes calls to stop family unification of refugees already in Germany, strip immigrants convicted of "significant crimes" of their German passports, and declare Islam incompatible with German culture.
But commentators said the power struggle further undermined its bid to surf the momentum of France's far-right frontrunner Marine Le Pen, Donald Trump in the United States and the Brexit movement in Britain to electoral success in the September 24 vote.
Spiegel Online journalist Severin Weiland said Saturday it was now even "doubtful" whether the AfD would clear the five-percent hurdle to representation in the national parliament.
"Frauke Petry was the public face of this party," he said.
Petry's chief rival, 76-year-old Alexander Gauland, a hardline defector from Merkel's CDU, had urged delegates to defeat her Realpolitik motion, calling it "divisive".
But even Gauland, who was widely mentioned as a candidate to join the AfD campaign team, expressed regret that Petry, who is very popular with the party's base, will not be front-and-centre on the campaign trail.
Another likely member of the election team is 38-year-old economist and former investment banker Alice Weidel who has railed against "an army of millions of uneducated migrants from the Middle East and Africa who expect a free ride" in Germany.
The daily Süddeutsche Zeitung said that the dispute was less about the political goals of the party, the most successful right-wing populist outfit in Germany's post-war history, than personal ambition.
"The AfD is heading for a showdown that could end up breaking it apart," it said.