"We were getting reports of Sinn Fein going twice to the same doors on the day of the election asking if people had voted," she said.
"People were complaining to us that there were up to 10 men standing outside the voting station and that they found it quite intimidating."
She said there were also cases of republican ballads being blasted at SDLP candidates when they arrived at polling stations.
SDLP MLA for Newry and Armagh Dominic Bradley had a similar story: "Some people I know of were canvassed up to four times during the course of the campaign," he said.
"I think they were selected. In other words, anyone that was not convinced, they would revisit."
One nationalist source said she saw Sinn Fein members holding an elderly man's hand and putting the voting card into it to show him who he should vote for.
She said party members had a colour-coded register defining whether voters were definite Sinn Fein, possible Sinn Fein, SDLP and those who didn't vote last time.
The "marked register", which is available to all parties, states whether or not individuals voted, but not for whom anyone voted.
"Every person was marked off with a colour," she said. "The elderly found it slightly intimidating. I overheard one man, in his late 70s, saying to his wife 'do we have to vote for them?'
"We also heard of someone getting a visit from three party workers in their own living room, quizzing them on how they would vote."
A republican source in Newry and Armagh said Sinn Fein election day operations are run from caravans outside voting stations, with cars being sent to people's doors if they have not voted.
He disagreed with parties getting the marked register, showing whether each individual had voted or not in the previous election.
"For the supposed Sinn Fein voters who didn't vote there will be a follow-up visit asking the question: 'What was the problem, why did you not vote?'" he said.
"Personally, I think it is intimidatory. It sends out a subliminal message – we are 'big brother' and we are watching you."
A Sinn Fein spokesman hit out at the criticism, saying the SDLP was simply jealous of Sinn Fein's success in the election.
He said Sinn Fein carried out its own tally on who had voted through volunteers outside each polling station as the election day progressed.
"The SDLP would like to do this but doesn't have enough management," he said.
"The elections involve a secret ballot so you can't be intimidated. It isn't credible and it has to be remembered that Sinn Fein has never been convicted of electoral malpractice," he added.
A spokesman for the Electoral Office said it was content with procedures during the election and had not received any complaints.