THE SDLP's Margaret Ritchie has branded fake election letters distributed in her name as "dirty tricks".
It is understood the letters, which included a false SDLP letterhead, were posted to some people in South Down and appear to have been targeted at unionists.
The constituency is expected to be one of the closest-fought in the north, with Ms Ritchie battling with Sinn Féin's Chris Hazzard in a bid to retain her seat.
The SDLP has reported the letters to police and the Electoral Commission.
The fake election literature suggests the SDLP wants a border poll to take place after Brexit.
"People from the Unionist tradition should not fear the prospect of a united Ireland in the near future, especially with politicians like myself representing you, I can represent you better than any Unionist," the letter claims.
Ms Ritchie said it was an "attack on the community in South Down and on the integrity of our democratic process in this Westminster election".
"I have been an MP for everyone in South Down," she said.
"Anyone who knows me will know that my door is open to those of all faiths, political and social backgrounds. My campaign has been based on my record of delivery for local people, ensuring that they have strong representation and real representation at home and at Westminster."
She added: "People in South Down will see through these dirty tricks. They know my character and my passion for all the people in south Down and we will not be deterred."
Meanwhile, independent unionist councillor Henry Reilly has called on unionists not to vote for the SDLP in the key constituencies of South Down and Foyle.
Mr Reilly tweeted the results of a general election poll, carried out by Survation, which showed Labour down just one per cent behind the Conservatives.
He tweeted: "This is getting out of hand - do not vote SDLP in South Down or Foyle - two shinners elected is good for us. THINK.."
Mr Reilly said last night while he would never endorse a vote for Sinn Féin, he said it "just did not make sense" for unionists to vote for SDLP candidates in closely-fought nationalist constituencies.
"I wouldn't usually comment on any battles within nationalism," he said. "But this is the most extraordinary election I have ever been involved in and I've been involved in quite a few."
Mr Reilly said he feared in the event of a hung parliament, the SDLP could join the SNP to prop up a possible Labour government.
"The Tories and Labour will offer anything to anyone (other party) to get their votes," he said.
"The SDLP could be very influential and could ask for a border poll."
The Mournes councillor said while Sinn Féin do have close links to Labour leader Jeremy Corbyn, their influence would be limited because they do not take their seats at Westminster.
"For unionists it makes sense to have fewer nationalists sitting in Parliament," he said.
