In one of his most explicit attacks on US Democratic presidential candidate Barack Obama to date, Republican candidate Sen. John McCain challenged his opponent's ability to handle security threats, implying in a television ad that Obama would endanger the world and Israel by underestimating Iran."Obama says Iran is a 'tiny' country," the ad begins, with ominous images of Iran and its leader, President Mahmoud Ahmadinejad, in the background. "'Doesn't pose a serious threat.' Terrorism, destroying Israel, those aren't 'serious threats'?"The rhetorical question is then followed with the bottom line: "Obama - dangerously unprepared to be president."Double click on play icon below to watch adThe ad appears to use a direct quote made by McCain's Democratic rival in which he discussed Iran in the context of the Cold War."Strong countries and strong presidents talk to their adversaries. That's what Kennedy did with Khrushchev. That's what Reagan did with Gorbachev. That's what Nixon did with Mao," Obama said in May. "I mean think about it. Iran, Cuba, Venezuela - these countries are tiny compared to the Soviet Union. They don't pose a serious threat to us the way the Soviet Union posed a threat to us. And yet we were willing to talk to the Soviet Union at the time when they were saying, 'We're going to wipe you off the planet.'"In response, the Obama campaign issued a statement calling the ad a distortion of the truth."John McCain is distorting Barack Obama's words to cover up for the fact that it's the failed Bush-McCain approach to foreign policy and the Bush-McCain war in Iraq that have strengthened Iran and endangered Israel," said Obama campaign spokesman Hari Sevugan. "If John McCain was serious about dealing with the threat from Iran, he would join Barack Obama's bipartisan effort in the Senate to step up sanctions on Iran instead of adopting the same tired, old Bush-Rove playbook."