Arlene Foster has launched a scathing attack on outgoing DUP MLA Jim Wells after he declared his support for a TUV Assembly election candidate.
She accused Mr Wells of never accepting her leadership of the DUP as a woman, saying he was "still living in the 1970s".
The former DUP leader said the party selecting a "capable unionist woman" to contest the South Down constituency in May was "too much for Jim to bear".
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Mr Wells rejected Mrs Foster's remarks, saying she has never forgiven him for supporting her being ousted as DUP leader.
The outgoing MLA, who has been the DUP's representative in South Down since 1998, has quit the party after endorsing TUV candidate Harold McKee for the constituency.
The former Health Minister had been deselected as the DUP candidate in the area.
He previously endorsed an attempt by the DUP's Edwin Poots to switch from Lagan Valley to South Down, but refused to back Diane Forsythe after she was chosen instead by party officers.
Mr Poots later switched to contest South Belfast after replacing party MLA Christopher Stalford following his sudden death.
Mrs Foster quit as DUP leader last year after an internal party revolt and Mr Wells backed Mr Poots to succeed her.
In a series of tweets, Mrs Foster criticised Mr Wells and endorsed Ms Forsythe's candidacy.
The former First Minister tweeted: "If Jim Wells' mate Edwin Poots had been selected to run in South Down Jim would not be supporting the TUV candidate.
"Jim didn't get his own way and so has thrown his toys out of the pram and gone off with the boys.
"The fact that Diane Forsythe is a capable unionist WOMAN is too much for Jim to bear."
She added: "Just as he could never accept the fact that I was party leader as a WOMAN and he continuously agitated against me.
"Jim is still living in the 1970s - sad but there it is.
"The voters in South Down have a choice - principled, able, active representation in Diane Forsythe (something they haven't had for years with Jim Wells) or someone backed by a Jim Wells who thought he could impose his will on the voters of South Down as if the seat was his and not theirs.
"I believe in the sense and sensibility of ordinary voters who individually, all have equal power on May 5. Good luck @dianejforsythe and all those who are in politics not for self but the country. #AE22 #ThisGirlCan."
In response, Mr Wells strongly rejected Mrs Foster's comments and expressed support for having more women in political life.
He noted that when asked in an interview some years ago to name a politician he admires outside of the DUP, he chose the SDLP's Claire Hanna.
Mr Wells added: "I have never had the slightest difficulty with any women on a personal basis. I have two daughters.
"I was opposed to the candidate imposed on South Down regardless of them being male or female.
"It is her policies rather than the fact that she is female, so I do resent what Arlene has said."
He added: "I think what is behind this is Arlene has never forgiven me for being supportive of her removal as leader."
Earlier Mr Wells publicly endorsed TUV South Down candidate Mr McKee, describing him as a "politician of conviction, not expediency".
He said: "As I leave the Assembly it is my earnest desire that South Down should continue to have a unionist MLA whose politics are grounded in conviction and principle.
"Among the candidates Harold McKee is the standout candidate with these credentials."
Mr McKee, who in 2016 briefly served as the UUP's South Down MLA, quit the party last year after saying his Christian beliefs were at odds with Doug Beattie's leadership.
The Newry, Mourne and Down councillor hit out at Mr Beattie's stance on abortion, same-sex marriage and a vote at Stormont on gay conversion therapy.
He subsequently defected to TUV.
Mr McKee expressed his delight at receiving an endorsement from Mr Wells and praised his "decades of faithful service" in the South Down constituency.
TUV leader Jim Allister also welcomed the support of Mr Wells, saying he was "putting principle before party".
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