Sea levels are rising while glaciers and summer Arctic sea ice are shrinking. Plants are blooming earlier. Some species could die because of global warming.
Obama proposed a bill to cap power plant carbon dioxide emissions, but it died in Congress. Still, he's doubling auto mileage standards and put billions into cleaner energy. Romney now questions the science of man-made global warming and says some actions to curb emissions could hurt an already struggling economy.
The risk of a devastating cyberattack on the United States is real. Yet the country remains vulnerable to an electronic Pearl Harbor due to a political dispute over the role the federal government should play in securing the computer networks that control the electrical grid, water supply and other critical sectors.
Obama wants the owners of essential U.S. infrastructure to meet minimum cybersecurity standards. But Republicans in Congress say the president's approach will only lead to costly, time-consuming regulations that won't reduce the risk. Romney says Obama has failed to lead on a critical national security issue.
While Congress bickers, the Pentagon worries. "The uncomfortable reality of our world today is that bits and bytes can be as threatening as bullets and bombs," Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told lawmakers.
A sea of red ink is confronting the nation and presidents to come.
The budget deficit — the shortfall created when the government spends more in a given year than it collects — is on track to top $1 trillion for the fourth straight year. The government borrows about 40 cents for every dollar it spends.
The national debt is the total amount the federal government owes. It's risen to a shade over $16 trillion.
Obama has proposed bringing deficits down by slowing spending gradually, to avoid suddenly tipping the economy back into recession. He'd raise taxes on households earning more than $250,000 and impose a surcharge of 30 percent on those making more than $1 million. Romney would lower deficits mostly through deep spending cuts. But many of the cuts he's pushing would be partially negated by his proposals to lower top tax rates on corporations and individuals.
At its core, the debate over how much the U.S. spends on defense gets down to this: What is it that America should be defending against?
There are plenty of potential security threats on the horizon, not to mention an unfinished war in Afghanistan.
The size and shape of the defense budget go a long way toward determining whether the U.S. can influence events abroad, prevent new wars and be ready for those it can't avoid. It also fuels the domestic defense industry in ways that affect the vitality of communities large and small across the country.
Obama wants more restraint in military spending while Romney favors expansion. Obama also wants more focus on Asia-Pacific security, reflecting China's military modernization. But that and other elements of military strategy could come apart if Washington doesn't find a way to avoid automatic budget cuts starting in January.
The job market is brutal and the economy weak. More than 12 million Americans can't find work; the unemployment rate fell in September but is still at a recession-level 7.8 percent. It had been more than 8 percent for 43 straight months. A divided Washington has done little to ease the misery.
The economy didn't take off when the recession ended in June 2009. Growth has never been slower in the three years after a downturn. The human toll is staggering. Forty percent of the jobless, 4.8 million people, have been out of work six months or more — a "national crisis," according to Federal Reserve Chairman Ben Bernanke. Wages aren't keeping up with inflation.