The entire officer team - who represent 305 years membership of the party’s South Down Association have - announced they will be instead supporting the TUV’s Harold McKee.
He said that other members “are either going to join the TUV or are thinking about it”.
“But I cannot speak for them,” he added.
A statement from the South Down Association of the DUP said: “All of the DUP South Down officers resign from the Party and pledge their support to TUV candidate Harold McKee.
“Those resigning include the current Chairman, Vice Chairman, Secretary, two founding members of the DUP and several former councillors.
“This very significant move follows the recent decision of Jim Wells – The DUP MLA for South Down since 1998 - to publicly endorse Councillor McKee.
“Mr Wells has made it clear that he could not support the DUP candidate, Diane Forsythe, who he believes was imposed on the local members and has publicly stated that Harold McKee has the experience and views on social issues which make him the best candidate to represent South Down.”
Chairman of the South Down Association, Roland Wilson said: “I have been a loyal member of the DUP for over 40 years but like many other supporters of the Party I cannot under any circumstances support Diane Forsythe.
“Not once in the entire selection process was I or any other member of the DUP South Down Association consulted about this crucial decision. Every letter, email and text message we sent to Party Headquarters about this issue was totally ignored.
“The Party asks our members to raise money to fund the central activities of the DUP, pay election expenses, put up posters and knock doors and the least we expect in return is that one of our communications about such an important issue was answered.
“Local members have been treated with disdain and had a candidate imposed upon us who we could never endorse.
“Loyal DUP members of the Party in South Down are extremely fortunate that we do have a candidate standing on 5th May who we can enthusiastically support. Councillor Harold McKee has the experience at both local government and Stormont level to effectively represent the constituency in the Assembly and we fully endorse his recent stand on moral issues such as the protection of the unborn child and same sex marriage.”
TUV leader Jim Allister said that “the continuing groundswell of support for Harold McKee is very gratifying”.
“It is now clear that Harold is now the candidate of choice for principled Unionists right across the spectrum,” he added.
“The coalition of support that he is building is the foundation for a great victory for Unionists of conviction in South Down on May 5th. I thank all who have rallied to support him’.
“Those resigning include Roland Wilson, the Chairman of the South Down DUP Association, Garth Craig, a former Chairman of the Association and Councillor on Down District Council and Newry Mourne and Down Council, William Burns, the Association Secretary and a member of Newry and Mourne Council for 24 years, Fergus Bingham, a foundation member of the Party when he joined in 1971, Clifford Wilson, a member for 20 years and previously active in the Young Democrats – the youth wing of the Party, David Herron – current Vice Chairman of the South Down Association and former Banbridge District Councillor for over 30 years and a former Chairman of the Council, and Olive Craig – a Party member for over 40 years.”
A DUP spokesman said: “Diane is leaving a professional career to contest this election and is best placed to win the one unionist seat in South Down.
“This is reflected by the influx of new members into the South Down Association since her selection as well as the reengagement of many longstanding members in support of her.
“We thank those who have resigned for their service to the Party but remind them that the TUV can’t stop SF’s divisive border poll plans. Only the DUP can win. On the doorsteps unionist voters across South Down are crying out for unity.
“Further splits and rancour in unionism serves no useful purpose. A first preference vote for any other Party in South Down could result in Sinn Fein winning the election, giving them the mandate they have always wanted to advance a divisive border poll.
“Voters are uniting behind Diane to ensure a unionist victory in this election.”