Sinn Féin Senator Trevor Ó Clochartaigh has quit the party over what he described as inaction in dealing with disciplinary issues in his Galway West constituency.
He said there have been serious breaches of the Sinn Féin code of conduct that have gone unpunished.
The Senator, first elected to the Seanad in 2011, also spoke about unacceptable behaviour against him.
He said: "We have had serious breaches of the Sinn Féin code of conduct in Galway West going unpunished and unacceptable behaviour against me and a number of other members locally, from a small number of ruthless, unscrupulous and ambitious individuals going back a number of years now, which has no place in a modern political party."
He said this matter was raised with the party leaders’ office yesterday, but there has been no resolution.
Mr Ó Clochartaigh also criticised a decision to "plough ahead" with a selection convention in the constituency, which he alleged had a pre-determined outcome.
In response, Sinn Féin said it was extremely surprised at Mr Ó Clochartaigh’s decision.
It said that it had decided to go for a one-candidate strategy at its selection convention this weekend.
It claimed that Mr Ó Clochartaigh was looking for a clear run at the convention.
"It is clear that Trevor was worried that a democratic vote of party members in the constituency would not select him to contest the election," the party said.
"It is clear that Trevor was looking for a clear run at convention and the party wasn't prepared to bypass our democratic structures to allow for that.
"Given he was arguing just yesterday for a two-candidate strategy, which he would have been happy to be part of, we are extremely surprised at his decision to leave the party today."
Sinn Féin Senator Trevor Ó Clochartaigh announces his resignation pic.twitter.com/L2LVUqV5d7— RTÉ News (@rtenews) November 30, 2017