Tehran can disrupt global fuel supplies, hit U.S. allies in the Gulf or support proxies such as Hezbollah in acts of terrorism. It could also draw the U.S. into an unwanted new war in the Muslim world.
Supreme Court appointments:
With four justices in their 70s, odds are good that whoever wins in November will fill at least one Supreme Court seat. The next justice could dramatically alter the direction of a court split between conservatives and liberals.
One new face could mean a sea change in how millions get health care, shape gay rights and much more.
Obama already has put his stamp on the court by selecting liberal-leaning Justices Elena Kagan and Sonia Sotomayor, 50-somethings who could serve a quarter-century or more. Romney has promised to name justices in the mold of the court's conservatives.
Since the New Deal, Supreme Court decisions have made huge differences in American lives, from rulings to uphold Social Security, minimum wage laws and other Depression-era reforms to ringing endorsements of equal rights. Big decisions on health care, gun rights and abortion have turned on 5-4 votes.
Unless Congress acts, the trust funds that support Social Security are on pace to run out of money in 2033, triggering an automatic 25 percent cut in benefits that millions of older Americans rely on for most of their income.
That may seem far off. But the sooner Congress acts, the more time to phase in changes slowly.
Social Security could be preserved for generations with modest but politically difficult changes to benefits or taxes, or some of both.
Obama hasn't laid out a detailed plan for addressing Social Security. Romney proposes a gradual increase in the retirement age and, for future beneficiaries, slower growth in benefits for the wealthy.