VIENNA (Reuters) - Austria s ruling Social Democratic Party (SPO) is suing Foreign Minister Sebastian Kurz, the chairman of its junior coalition partner, after he accused Chancellor Christian Kern s party of secretly accepting a 100,000 euro ($119,120) donation. The lawsuit marks a new low in the already strained relationship between the coalition partners, who have engaged in public squabbles for years. Polls for parliamentary elections scheduled for Oct. 15 show Kurz s conservative People s Party (OVP) ahead of the SPO.     The lawsuit relates to an interview shown on public broadcaster ORF on Monday in which Kurz said industrialist Hans-Peter Haselsteiner, a vocal supporter of the pro-business Neos party, donated 100,000 euros to the SPO via opaque channels. The lawsuit filed at Vienna s commercial court on Friday said the SPO s  good reputation, economic progress and honor was damaged  by Kurz s comments as it implied dishonesty. The SPO wants party donations to be capped at 20,000 euros.  With a view to the upcoming parliamentary elections, there is a very concrete danger that the (SPO) will achieve a worse result because of this accusation,  the lawsuit, seen by Reuters, said. The SPO said it has not been offered or received any donations from Haselsteiner, who is a key shareholder of construction group Strabag. Haselsteiner denied Kurz s accusations in newspaper Der Standard. The SPO wants Kurz to publicly retract his comments. In an emailed statement, the General Secretary of Kurz s OVP said Haselsteiner had donated money to an association that is critical of the OVP and hence indirectly supports the SPO. She said the SPO should make all its donations public. Kurz s spokesman was not immediately available for comment. 