President Barack Obama's accession to the presidency is truly inspiring. Today, every American boy and girl knows that there is nothing they cannot achieve if they apply themselves. This powerful sense of hope extends well beyond America's shores as people throughout the world try to bring the same optimism to their own countries. But Obama also takes office at a time when America confronts enormous economic and foreign policy challenges. He will have to marshal all his considerable leadership skills to address these dual challenges. He has wisely chosen to surround himself with a first-rate leadership team and, no less important, he has also inspired confidence in his countrymen in the possibility of change and in the hope of a better future. The challenges America's new president faces are indeed immense. The acute financial crisis this past fall and the sharp slowdown of the real economy that followed it, makes a prolonged and deep recession in the United States a distinct possibility. Obama himself has already warned of the possibility of double-digit unemployment in the year ahead. In foreign policy, Obama faces a wide array of difficult decisions, from how to responsibly withdraw from Iraq to how to advance peace between Israelis and Palestinians. But the outcome of one issue will prove more important to Obama's presidency than all others: Will his administration succeed in preventing Iran from acquiring nuclear weapons? A nuclear-armed Iran will change the world as we know it. It will pose a direct existential threat to Israel. Equally, Iran's terror proxies, including Hezbollah and Hamas, will operate under an Iranian nuclear umbrella. Iran will move quickly to dominate the world's oil supplies and the nuclear nonproliferation treaty will be rendered meaningless. I am convinced that Obama recognizes these dangers. When he visited Jerusalem last summer, he said that the United States cannot afford a nuclear-armed Iran. I believe that Obama is working from his first day in office to thwart Iran's nuclear ambitions. LIKE AMERICA, Israel faces enormous challenges. We too are not immune from the economic turbulence sweeping the world. We also must take immediate action, including enacting sharp tax cuts and implementing bold economic reforms, to protect jobs and sustain growth. Our security challenges are no less daunting. Hamas remains in power and will try to rearm itself with an even more deadly arsenal. Hizbullah has de facto control over Lebanon and has tripled its lethal capacity. And advancing peace with moderate Palestinians is possible, but must be done in a way that does not sacrifice Israel's security interests. Above all else, the top priority of the next government of Israel will be to ensure that Iran does not acquire nuclear weapons. Iran is a regime openly pledged to our destruction, and its threats must never be dismissed lightly. Israel must immediately redouble its efforts to work with the United States and other allies to neutralize this threat. In three weeks time, Israelis will be able to choose a leadership that can address the difficult challenges we face. I believe that my colleagues in the Likud and I can provide that leadership. The writer is head of the opposition in the Knesset and chairman of the Likud Party.