Obama achieved historic increases in fuel-economy standards and imposed the first regulations on heat-trapping gases blamed for global warming. His administration tightened standards on mercury pollution from power plants and set new controls on soot.
But he couldn't persuade a Democratic Congress to pass limits he promised on carbon emissions and shelved a plan to toughen health standards on lung-damaging smog.
Romney questions the cause of climate change and he's criticized Obama's treatment of coal-fired power plants. He opposes treating carbon dioxide as a pollutant and wants the cost of complying with regulations given more consideration.
European economic crisis:
Europe is struggling to control a debt crisis, save the euro currency and stop a repeat of the 2008 financial crisis that sent the world into recession.
Europe's troubles are the No. 1 threat to the U.S. economy. The biggest fear is that the 17-country eurozone will split, causing a financial crisis that will spread across the Atlantic, freeze credit and send the U.S. economy back into recession.
Neither Obama nor Romney has offered plans for Europe. The U.S. government lacks the cash and the will to rescue European countries struggling with huge government debts.
Obama has urged Europe to act more decisively. Romney warns that the United States will face its own day of reckoning if it can't reduce the federal debt. Many economists call for eurozone countries to assume joint responsibility for the weakest countries' debts through eurobonds; Germany has balked at the idea.
Both sides of the gay marriage debate agree on this much: The issue defines what sort of nation America will be.
Half a dozen states and the District of Columbia have made history by legalizing it, but it's prohibited elsewhere and 30 states have placed bans in their constitutions.
Obama supports legal recognition of same-sex marriage, as a matter decided by states. Romney says same-sex marriage should be banned with a constitutional amendment.
The debate divides the public down the middle, according to recent polls, and stirs up passion on both sides.
In November, four states have gay-marriage measures on their ballots. In Minnesota, the vote is whether to ban gay marriage in the state constitution. Voters in Maine, Maryland and Washington state are voting on whether to legalize gay marriage.
Thus far, foes of gay marriage have prevailed in all 32 states where the issue reached the ballot.
Gun violence has been splayed across front pages with alarming frequency lately: the movie theater killings in Colorado, the Sikh temple shootings in Wisconsin, the gunfire outside the Empire State Building and more. Guns are used in two-thirds of homicides, according to the FBI. But the murder rate is less than half what it was two decades ago.
Neither Obama nor Romney has had much to say about guns during the campaign. Obama hasn't pushed gun control measures as president; Romney says new gun laws aren't needed.
It's getting harder to argue that stricter gun laws are needed when violent crime has fallen by 65 percent since 1993.
But the next president may well fill at least one Supreme Court seat, and the court is narrowly divided on gun control. An Obama appointee could be expected to be friendlier to gun controls than would a Romney nominee.
America's health care system is unsustainable. It's not one problem, but three: cost, quality and coverage.
The U.S. has world-class hospitals and doctors. But it spends far more than other advanced countries and people aren't much healthier. And in an aging society, there's no reliable system for long-term care.
Obama's expansion of coverage for the uninsured hits high gear in 2014. Obama keeps today's Medicare while trying to slow costs. He also extends Medicaid.