"I don't know if you follow the story of A Q Khan, the Pakistani nuclear scientist who stole technologies from Europe, brought it back. He's a national hero in Pakistan," Jeb Bush, son of former president George H W Bush and brother of ex-president George W Bush, said on Thursday.
Khan, 78, was placed under house arrest in 2004 in Pakistan after he confessed to running a clandestine nuclear proliferation ring. The scientist later retracted his confession and authorities eased restrictions on him in 2009.
"He lives, you know out in the open. He created the capacity to do, just as you're describing, in Pakistan and was caught trying to export that technology to where? Iran and to Libya," 62-year-old Bush said at an event organised by the Chicago Council on Global Affairs.
Bush, the former Florida Governor, has announced his intent to run for the 2016 presidential polls.
He was responding to a question that if Iran wanted nuclear weapon, they would just go to one of the bad boys in the neighborhood, like North Korea, Pakistan or Russia.
"It's an argument that - that's also being used for proliferation, like then the Saudis will go and buy one too. And so, what you're describing isn't, you know, something, why would they do it? They've tried. They absolutely have tried," he was asked.
Citing Khan, Bush said such an effort has been made by countries like Iran in the past.
"The idea that other nations aren't trying to do this is - it's - look, we've got to be real about this. They are trying. And this is why engagement in the treaty and the proliferation efforts that are - that are global and ongoing to make sure that we retard any efforts in that regard are hugely important," he said.
"It just makes me more committed to an America that is fully engaged in the world, because as the rest of the world gets weak and more insular in their thinking because of their economic conditions, we have to be engaged because these things will occur," he said.
"There are other threats as well, but this one is probably the biggest in the globe right now," Bush said.