Democrat Barack Obama and Republican John McCain face the judgment of US voters on Tuesday after a long and bitter struggle for the White House, with Obama holding a clear lead in national opinion polls.

At least 130 million Americans are expected to cast votes on a successor to unpopular Republican President George W Bush and set the country's course for the next four years on the economic crisis, wars in Iraq and Afghanistan, an overhaul of health care and other issues.

The first polls begin to close in parts of Indiana and Kentucky at 6 pm EST (2300 GMT) on Tuesday. Voting ends over the next six hours in the other 48 states.

Obama, 47, a first-term senator from Illinois, would be the first black US president if elected. Opinion polls indicate he is running ahead of McCain in enough states to give him more than the 270 electoral votes he needs to win.

A victory for McCain, 72, 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 US vice president.

McCain, an Arizona senator, embraced his role as an underdog and says he is gaining ground on Obama. He was hitting seven states as he tried to pull off the biggest upset in recent political history.

'The pundits have written us off like they have before,' McCain said in Roswell, New Mexico. 'They may not know it, but the Mac is back. We're gonna win this election.'    

Both candidates hammered home their campaign themes in the final hours, with Obama accusing McCain of representing a third term for Bush's policies and being dangerously out of touch on the economy.

'When it comes to the economy, the truth is that John McCain has stood with President Bush every step of the way,' Obama said at his final campaign rally in Manassas, Virginia, a state Democrats have not won in a presidential election since 1964 but where Obama leads.

McCain, whose campaign has attacked Obama as a socialist and accused him of being a 'pal' with terrorists, portrayed him as a liberal who would raise taxes.

'He's in the far left lane of American politics and he's stuck there,' McCain said in Blountville, Tennessee.

Opinion polls show Obama ahead or even with McCain in at least eight states won by Bush in 2004, including the big prizes of Ohio and Florida. Obama leads comfortably in all of the states won by Democrat John Kerry in 2004.

Breakthrough victories in any of those traditionally Republican states -- including Virginia, Colorado, Indiana and North Carolina -- would likely propel Obama to the White House.

He took command of the race in the last month as a deepening economic crisis reinforced his perceived strengths on the economy, and in three debates where his steady performance appeared to ease lingering doubts for some voters.

In contrast, McCain has struggled to separate himself from Bush in a difficult political environment for Republicans, who are trying to hold the White House for a third consecutive term.

Democrats are also expected to expand their 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.

That would increase pressure on Democrats to produce on campaign promises to end the war in Iraq, eliminate Bush's tax cuts for the wealthy and overhaul a health care system that leaves 47 million Americans uninsured.

Obama choked up in North Carolina on Monday night when he talked about the death of his grandmother, Madelyn Dunham, who had helped raise him. Obama learned earlier in the day that Dunham had died of cancer in Hawaii, and he called her a 'quiet hero' like many other Americans.

Both presidential candidates planned more campaign stops on Tuesday. Ob