The motion, passed by 1,022 votes to 813 at the Green Party of England and Wales conference in Brighton, defines Zionism as "an ethnonationalist settler-colonial project" and supports the establishment of a single democratic state in historic Palestine. It has drawn sharp criticism from other British political parties.
Prime Minister Andy Burnham has ruled out partnering with the Greens, calling the motion "wrong" and "anti-Semitic". Several of the party's own left-wing MPs have also criticised the move.
Speitan said the threats were "anticipated" and that she believes they are a form of "intimidation" from pro-Israel agitators. While "no one wants to be on the receiving end of death threats", she said it is "necessary to confront these evils".
Speitan said 56 members of her family have been killed since Israel launched its military campaign in Gaza three years ago. She rejected accusations of anti-Semitism, stating the motion "targets an ideology: Zionism" and recognises that "not all Jews are Zionists and not all Zionists are Jews".
Recent media reports have claimed Speitan was "friends" with an "al-Qaeda terrorist" after she reportedly offered to testify in defence of a UK citizen who planned an attack at Heathrow Airport. Speitan described this as a "tactic of guilt by association" that she would not dignify with a response.
The Green Party told Al Jazeera that Speitan holds no party positions "other than being a member" and that it takes all allegations of this nature seriously.