For months, Renault SA's partnership with Nissan Motor Co. has been a strain on management and a public relations nightmare. Now the Japanese carmaker's woes are hitting where it really hurts: the bottom line.

The largest French automaker, which owns 43 percent of Nissan, reported Friday that its partner's poor performance in the first half cut net income by €21 million ($23.4 million). A year earlier Nissan added €805 million.

Nissan is smarting from slumping U.S. sales and aging vehicle models. The worsening results come at a time of heightened tension within the alliance with Renault in the aftermath of the arrest of their former chairman, Carlos Ghosn, who held the partnership together for almost two decades.