Sheila Coleman, who introduced Jeremy Corbyn's speech at Labour conference on Wednesday, nominated a candidate from a rival party as recently as April.
The campaigner and Unite member backed Daren Andrew Ireland from the Trade Union and Socialist Coalition (TUSC) in a Liverpool city council by-election, though the Labour candidate eventually won.
The activist is known locally for being a part of the Hillsborough Justice Campaign, and received a honorary doctorate from the Open University earlier this year for her work.
Coleman, who told members in the hall to unite behind their "socialist leader", recently deleted her Twitter account, but some of her tweets were screengrabbed beforehand.
In one tweet from 2014, she also said she had "never voted" and would never vote for her local Labour MP, Louise Ellman.
BuzzFeed News has reached out to Jeremy Corbyn's office for comment.
Marie Le Conte is a politics and media reporter for BuzzFeed News and is based in London.
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