A councillor who quit the Green Party to join the SDLP had just weeks earlier been considering a switch to Alliance.
Simon Lee is understood to have made an approach to Alliance within days of May's Stormont election, in which he ran unsuccessfully as a Lagan Valley candidate for the Greens.
Further discussions were held but the Lisburn and Castlereagh councillor some weeks ago decided against joining Naomi Long's party.
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It has also emerged Mr Lee has in the past expressed disapproval of the SDLP's partnership with Irish government party Fianna Fáíl.
In a tweet in January 2019 reacting to the move, he said: "This is madness right before an election. The fallout could cause serious damage."
Mr Lee was elected in May that year when a rise in support for the Greens saw the party double their council seat tally to eight.
Green Party sources noticed a clear-out of Mr Lee's social media accounts in recent days before his defection to the SDLP was announced.
Numerous past tweets in Twitter conversations that reference the SDLP appear to have been deleted.
The Greens suffered significant defeats in May's election, losing both their Stormont seats to an Alliance surge that saw the party more than double its number of MLAs to 17.
The SDLP also had a difficult election, dropping from third-largest party at Stormont to fifth-largest after losing four MLAs including deputy leader Nichola Mallon.
Now with eight seats in the Assembly, the party has taken up the role of the official opposition after losing automatic entitlement to a ministerial post in a new power-sharing Executive.
A party spokesman said: "The SDLP is aware that Simon gave careful consideration to which party best met his values as a public representative.
"The SDLP is built on values of reconciliation, social justice, defending workers' rights, enhancing public services so that everyone can access health treatment and a good quality job.
"We are delighted to have someone of Simon's calibre, character and values on our team."
In an earlier statement, Mr Lee said his move to the SDLP was "a big decision" but he felt the party was a "perfect fit" for him.
The councillor, a Presbyterian from a unionist background, said that "Brexit has changed everything" and in recent years he has become "more attracted to the party's work on building a new Ireland".
The Castlereagh South representative also told the Belfast Telegraph he considered Alliance "too right-of-centre".
SDLP leader Colum Eastwood tweeted that he was "delighted" Mr Lee was joining his party's ranks.
SDLP South Belfast MP Claire Hanna also said she was "absolutely chuffed" to welcome Mr Lee, who alongside the Greens had endorsed her Westminster election campaign in 2019.
Ms Hanna described the councillor as a "first-class public representative, an incredibly hard worker and a really lovely guy".
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