Speaking out on the issue for the first time, Ben Emmerson, UN special rapporteur on counter-terrorism and human rights, took fierce aim at the Republican candidate's refusal to condemn waterboarding.
"There is no doubt that the Romney administration would be able to claim — in the event of a Romney presidency — a democratic mandate for torture," Emmerson told The Canadian Press.
"That would put Romney as the first world leader in history to be able to claim a democratic mandate for torture."
Emmerson said he planned to raise his concerns in an address to the UN General Assembly later this month, adding the issue was too important for him to stay out of the presidential race.
In the security panic that followed the 9/11 terrorist attacks, former U.S. president George W. Bush endorsed "enhanced interrogation" techniques, including waterboarding. The practice involves almost drowning a suspect to soften them up and get them to reveal information.
For example, Khalid Sheikh Mohammed, one of the suspects now on trial in Guantanamo Bay for planning the attacks, was waterboarded 183 times, according to the U.S. Justice Department.
During a debate in 2007, Romney said he opposed torture in any form, but ducked the question of whether he viewed waterboarding as torture. He also said he wanted other terrorists to face the same kind of treatment as Mohammed did.
Last month, when asked directly if he believed waterboarding to be torture, he responded, "I don't."
Romney has criticized his Democrat rival, President Barack Obama, for unequivocally ruling out the use of waterboarding. The Republican's advisers are also pushing him to campaign on scrapping the president's ban on such techniques, the New York Times has reported.
Emmerson, who was in Toronto to attend a symposium on the negative impacts post-9/11 security measures have had on human rights, said any attempt to label waterboarding as inhumane or degrading treatment — but not as torture — was pure "sophistry."
Obama, he said, had begun to realign U.S. policy with international law and the universal abhorrence of torture, and Romney's approach was threatening to undermine that progress.
Counter-terrorism experts maintain that American abuse of detainees in places such as Guantanamo Bay and Abu Ghraib in Iraq, or torture of suspects done by others for the U.S., has been a boon for terrorist recruiters.
Terrorists have also used the abuses to justify attacks on Americans around the world.
As such, while Bush, Romney and others have defended enhanced interrogations as critical to U.S. national security, Emmerson was adamant that going back to any form of torturing suspects would be dangerously counterproductive.
"The re-introduction of torture under a Romney administration would significantly increase the threat levels to (Americans) at home and abroad," Emmerson said.
"Such a policy, if adopted, would expose the American people to risks the Obama administration is not currently exposing them to."
In addition, much of the world is moving away from accepting or using intelligence that may be tainted by torture, U.S. intelligence-gathering efforts could be threatened if Romney had his way, he said.
Emmerson also took aim at the lack of transparency surrounding Obama's use of drones to target terrorists, and the Canadian government's 12-year detention of Mohamed Mahjoub on a national security certificate based at least in part on Egyptian evidence obtained by torture.