Brian Gallant said Tuesday that the pamphlet being distributed by the Canadian Centre for Bio-Ethical Reform was inappropriate and offended many of those who received it in their mailboxes.
"During an election campaign, it is normal for people to voice their opinions in order to influence voters and politicians," he said in a statement.
"But this material crosses a line. It is deeply offensive."
The double-sided card on the group's website appears to show a bloodied fetus and small body parts next to a picture of a smiling Gallant. It says, "A vote for Brian Gallant and the Liberals is a vote for this."
Alex vande Bruinhorst, a spokesman for the Canadian Centre for Bio-Ethical Reform, said the group targeted Gallant because it fears he will clear the way for taxpayer-funded abortion if the Liberals win Monday's election.
He said they have distributed about 5,000 postcards and plan to mail out another 20,000 before election day.
Vande Bruinhorst said they have used similar images to target politicians of various political stripes to raise awareness about abortion.
"We want to turn people off to abortion and we have evidence that the images do just that," he said.
"When you expose the reality of what abortion is ... although that's not pleasant to look at, we see that people's minds do change."
Gallant has said he would review any barriers to abortion, including a regulation that requires a woman who wants the procedure covered by medicare to have two doctors certify that it is medically necessary.
Premier David Alward has said he would maintain the status quo.
A spokeswoman for Alward said he had seen the postcards, but did not support them and was not involved in their distribution.
The development came as people in the province prepare to go to the polls following an election campaign that has focused largely on varying ideas for economic development and job creation.
Alward focused again on innovation and employment as he campaigned in Moncton. The premier repeated his platform promise to spend $18 million over the next four years on the province's knowledge sector if the Progressive Conservatives are re-elected Monday.
The Green party campaigned on access to health care.
David Coon said his party would open two community health centres each year over four years at a total cost of $28 million, bringing doctors, nurse practitioners, social workers, physiotherapists and others under one roof.
Gallant visited a daycare in Saint John to repeat his promise to add 6,000 daycare spaces and make them more affordable for low-income families.
Meanwhile, Elections New Brunswick says almost 70,000 people cast ballots during two days of advanced polling, up from just over 49,000 people who used advance polls in 2010.
Close to 13,000 people have also voted at their returning offices, and Elections New Brunswick has set up special polls for students at 13 campuses across the province.