Sen. John McCain encourages supporters after casting his vote on November 4, 2008 in Phoenix, Arizona. (Doug Pensinger/Getty Images)
McCain had targeted Pennsylvania, won by Democrats in the last four elections, as his best hope of stealing a Democratic-leaning state, but Obama was projected by television networks to win the state's 21 electoral votes fairly easily.
The loss leaves McCain with little margin of error in his quest for the 270 Electoral College votes needed to capture the White House. He must now win nearly all of the remaining battleground states -- all carried by President George W. Bush in 2004.
McCain and Obama were battling fairly evenly in Ohio, Florida, Virginia, North Carolina and Indiana as polls closed in more than half of the United States.
Long lines greeted voters in many key states but no major breakdowns or irregularities were reported as at least 130 million Americans are expected to cast votes on a successor to the unpopular Bush.
With about 10 million votes counted across the country, Obama narrowly led McCain. The two candidates traded wins in a series of states they were expected to carry.
Obama, 47, a first-term senator from Illinois, would be the first black U.S. president. Opinion polls indicated he was running ahead of McCain in enough states to give him more than the 270 Electoral College votes he needs to win.
A huge crowd of Obama supporters gathered in Chicago for an election night rally and cheered results that showed him doing well.
A victory for McCain, a 72-year-old former Vietnam War prisoner, would make him the oldest president to begin a first term in the White House and make his running mate Sarah Palin the first female U.S. vice president.
The winner will face a crush of challenges over the next four years, including the economic crisis, wars in Iraq and Afghanistan and an overhaul of the U.S. health care system.
After a campaign dominated by a debate over an economic crisis, exit polls showed 6 of every 10 voters listed the economy as the top issue.
Obama took command of the race in the last month as a deepening financial crisis reinforced his perceived strengths on the economy, and his steady performance in three debates helped ease lingering doubts among some voters.
Opinion polls before the voting showed Obama ahead or even with McCain in at least eight states won by Bush in 2004, including the big prizes of Ohio and Florida.
In U.S. presidential elections, each state is allocated a number of electors in the Electoral College. A candidate needs 270 Electoral College votes to win the White House race.
CAMPAIGN THEMES
The candidates hammered their campaign themes in the final hours, with Obama accusing McCain of representing a third term for Bush's policies and being out of touch on the economy.
McCain, whose campaign has attacked Obama as a socialist and accused him of being a "pal" with terrorists, portrayed him as a tax-raising liberal.
But in a difficult political environment for Republicans, McCain struggled to separate himself from Bush. Exit polls showed three out of every four voters thought the United States was on the wrong track.
In Ohio, Ian Edwards said he voted for Obama. "Very simple," said Edwards, the chief executive of a small technology company. "Bad war. Bad economy. Bad reputation overseas."
Casey Bradshaw, a cattleman in Canyon, Texas, said he voted early for McCain. "He's more experienced, and he's known to the people who make up this country," he said.
The race was closely watched around the world, including in Kenya, where in Obama's late father's village of Kogelo, residents prayed for his presidential bid and for his maternal grandmother, who died in Hawaii this week.
Democrats are also expected to expand majorities in both chambers of Congress. They need to gain nine Senate seats to reach a 60-seat majority that would give them the muscle to defeat Republican procedural hurdles.
Democrats scored two big victories early, knocking off two-high profile Republicans -- North Carolina Sen. Elizabeth Dole, a former presidential candidate and wife of 1996 Republican presidential nominee Bob Dole, and New Hampshire Sen. John Sununu.
