Police are appealing for information after finding the woman in her 20s with life-threatening injuries in Southwark, south London, at about 3am.
In a statement on its Facebook page, the Taking the Initiative Party said Sasha Johnson had been subject to numerous death threats.
The statement said: “It is with great sadness that we inform you that our own Sasha Johnson has sustained a gunshot wound to her head.
“Sasha has always been actively fighting for black people and the injustices that surround the black community, as well as being both a member of BLM and a member of Taking the Initiative Party’s executive leadership committee.
“Sasha is also a mother of three and a strong, powerful voice for our people and our community.
“Let’s all come together and pray for Sasha, pray for her recovery and show our support to her family and loved ones.”