Labour MPs who have defected to The Independent Group will be targeted by Momentum in the coming weeks.
The grassroots organisation that helped propel Jeremy Corbyn to the leadership will stage events with “celebrities” in the constituencies of those who have split.
Laura Parker, Momentum’s national coordinator, slammed the defectors, including Chuka Umunna, for “working hand-in-hand with the Tories”.
On Wednesday the eight Labour MPs who have already quit were joined by Conservatives Anna Soubry, Sarah Wollaston and Heidi Allen.
Momentum claims to have raised more than £15,000 in small donations from members in the hours after the Labour MPs quit on Monday.
The group plans to crowdfund tens of thousands of pounds more to fund its canvassing events in splinter group constituencies across the country.
Events will be held in Streatham, Stockport and Penistone and Stocksbridge, the constituencies of Umunna, Ann Coffey and Angela Smith.
The Labour leadership has demanded the MPs who defected trigger by-elections in their constituencies.
And the party has fast-tracked the selection process for Labour candidates in the seats held by the splitters.
Labour has also said the power of recall should be expanded so voters can try to force MPs out of their seats if they switch parties.
In a sign of the acrimony between the splitters and their old party, Labour dramatically shut down its confidential voter canvassing system on Wednesday amid claims one of the eight breakaway MPs tried to access party supporters’ personal data.