The agreement was signed by IDS secretary-general Tedi Chiavalon and the president of the HNS Istria branch, Veljko Ivancic, with the IDS candidate for Istria prefect, Valter Flego, present.
Chiavalon said the IDS would sign several more such agreements in the weeks ahead, including with the Italian Union and other national minorities in the county.
The agreement was signed two days after the HNS Presidency supported Flego's candidacy for county prefect.
Asked how the IDS saw the Croatian Pensioners' Party's (HSU) support to Damir Kajin as county prefect, Chiavalon said every party was entitled to choose their partners.
All three parties are members of the ruling coalition on the state level.