A reformist challenger to Iranian President Mahmoud Ahmadinejad, moderate cleric Mahdi Karroubi, said on Saturday that the hard-liner's denial of the Holocaust is serving Israel's interests and pushing the country deeper into international isolation. Karroubi is one of two reformist candidates hoping to unseat Ahmadinejad in the June 12 presidential election. Karroubi has vowed to bring change to Iran, saying Ahmadinejad has needlessly antagonized the West by claiming the Holocaust is a myth. He has also accused the president of failing to improve living standards despite huge oil revenues. In remarks published Saturday, Karroubi said attempts by Ahmadinejad to engage former US President George W. Bush and President Barack Obama in written letters got no response.