BERLIN — Germany’s center-left Social Democratic Party made an unusual show of solidarity Thursday with one of its biggest rivals by denouncing an advertising campaign targeting the environmentalist Greens.

The ads that appeared in several German cities less than two months before the country’s national election feature a wilted version of the Green party’s sunflower symbol, phrases such as “climate socialism” and “eco terrorism,” and the hashtag #GruenerMist, which means “Green Garbage.”

They were paid for by an organization called Conservare Communication, whose leader supported the far-right Alternative for Germany in the past. The party has denied involvement in the campaign.

The Social Democrats’ general secretary, Lars Klingbeil, commented on the posters in a tweet that read, “Green garbage is right-wing trash. Democrats stick together.”

Negative advertising is uncommon in Germany, but the Social Democrats recently received criticism for a video that attacked members of the election team backing Armin Laschet, the center-right Union bloc candidate for chancellor.

More than 60 million Germans are eligible to vote in the Sept. 26 parliamentary election that will determine which parties form the next government. Chancellor Angela Merkel is not running for a fifth term.

Merkel’s Union bloc is narrowly ahead of the Greens and the Social Democrats in recent polls.