As the NDP's foreign affairs critic, Dewar championed Arar's innocence and played a central role in demanding his release from a Syrian prison, where he was tortured for almost a year.
Ten years later, Arar is now returning the favour, throwing his support behind the Ottawa MP.
"Paul's knowledge of foreign policy and his involvement in supporting human rights causes in Canada and abroad have allowed him to grasp many of the complex issues that Canada and the rest of the world are facing today," Arar said in a written statement released Wednesday.
"I can easily picture Paul as the next prime minister of Canada."
In 2002, Arar was detained by American authorities during a stopover in the United States en route to Canada from a Tunisian vacation. The U.S. suspected Arar was a member of al-Qaida, the terrorist group behind the devastating attacks a year earlier on the World Trade Centre in New York and the Pentagon in Washington.
After holding Arar in detention for two weeks, without access to a lawyer, the U.S. deported him to his country of birth, Syria, despite his Canadian citizenship. He was imprisoned without charge and tortured for almost a year before finally being allowed to return to Canada.
A commission of inquiry subsequently concluded Arar was an innocent victim of faulty intelligence passed on to the Americans by Canadian security officials. The Canadian government eventually apologized and agreed to pay Arar $10.5 million in compensation. The RCMP also apologized for the injustice done to Arar.
His endorsement of Dewar marks the first time Arar has stepped into the realm of partisan politics, although his wife, Monia Mazigh, ran for the NDP in 2004. She has also endorsed Dewar.
Dewar said he was "shocked and deeply disturbed by the treatment of Maher Arar during those terrifying months he spent in Syria."
Should he become prime minister one day, Dewar added: "I promise to stand up and fight for the rights of all Canadians, regardless of religion, colour of skin or country of birth."
Arar joins a lengthy list of human rights advocates backing Dewar's leadership bid. Among others, Dewar has won endorsements from prominent human rights lawyers Paul Champ and Amir Attaran.
He has also won support form eight former staff at the troubled Rights and Democracy, a Montreal-based, federally-funded human rights agency that has been rocked by internal strife following several controversial appointments to the board by the Harper government.