Links to a newspaper article about Glasgow MSP Anne McTaggart, which reported she had been spoken to by police in a park at night while with a senior Glasgow councillor, were posted on Gail Casey's account.
Ms Casey, a West Dunbartonshire councillor, was selected on Wednesday to fight the Clydebank and Milngavie constituency in 2016.
The link on the social networking site was sent out about the same time as Ms Casey was made aware she would be on the extended list for the constituency and would be up against the married former Glasgow councillor.
Ms Casey, who is from a Labour dynasty in the Clydebank area, secured almost three times the number of votes as Mrs McTaggart in the party contest.
Despite her campaign being run by local MP John Robertson, Mrs McTaggart secured only five votes from more than 100 people on the night, with the rest being postal votes.
One source said of the twitter message: "It's hardly comradely behaviour."
Another insider said: "Gail would have known when the tweet was posted she was heading for the long list and that McTaggart would also. It was a case of getting the retaliation in first. She has wanted the seat for a long time and people were made aware of the infamous McTaggart story."
The message is one of only four posted on Ms Casey's Twitter account, the others all relating to Labour Party politics.
The story about Mrs McTaggart was published in a newspaper in late 2010 but only appeared on Ms Casey's account last August, almost three years later.
Ms Casey admitted the account was her own, but denied she had posted the story. She hung up the phone when asked how it had appeared on her account and why she had not removed it.
The selection meeting also saw former MSP Bill Butler returned to fight for the seat he lost by a handful of votes in 2011.
Mr Butler, who has been a Glasgow councillor since 2012, a year after he lost his seat, will stand in Anniesland, having won three times as many votes as his nearest rival, fellow former MSP Des McNulty.
But due to Labour's new strategy of twinning seats, with the candidate polling highest choosing his or her seat, senior party sources say the main contest was between Mr Butler and Mrs McTaggart.
Sources at the meeting described the "well-oiled machine" behind Mr Butler, adding that although the selection exposed deep divisions in the party in the west of Glasgow and factions against Mr Robertson, the successful candidate was seen as "being the most credible".
One said: "Anne had John Robertson behind her, and (former Glasgow City Council leader) Steven Purcell. This was not supposed to be a contest. But there were open mouths when the result came through. The 'chosen' candidate was nowhere to be seen, an indication perhaps of local divisions but also of her standing."
However, another said: "Bill Butler's selection sends out a message to the electorate that Scottish Labour has taken out a fatwa against talent."
Mrs McTaggart was unavailable. for comment.