U.S. President Barack Obama pulled ahead of Republican challenger Mitt Romney late Tuesday after winning California, Pennsylvania, Washington and Minnesota.

Obama was expected to take California and its 55 Electoral College votes, but he also won Pennsylvania, which was not a sure bet, and his late surge put him ahead 234 to 200.

Romney had been ahead for much of the evening, bolstered by wins in Texas, Arizona and North Carolina.

But hours after polls closed, a victor had yet to be declared in major battleground states such as Ohio, Virginia and Florida, which will likely determine who will win the White House.

Obama took Pennsylvania and its 20 Electoral College votes late Tuesday, but remained slightly behind Romney, who won Arizona鈥檚 11 electoral votes late in the day. Romney also won North Carolina and its 15 electoral votes late Tuesday.

Results posted shortly before 10 p.m. ET gave Obama Pennsylvania shortly after he won Michigan and its 16 electoral votes, as well as New York and its 29 votes and New Jersey鈥檚 14 votes. Romney took Texas as expected and its 38 electoral votes.

Romney attended a last-minute rally in Pennsylvania Tuesday afternoon in what turned out to be a fruitless bid to appeal to undecided voters in the state.

At 11:15 p.m. ET, Obama had 234 votes to Romney鈥檚 190.

As voters headed to the polls Tuesday, long lines at polling stations and concerns that some residents affected by Hurricane Sandy would be left disenfranchised raised the spectre of a protracted court battle like the one that marred the 2000 vote.

More than 45 million voters had cast ballots before election day in early voting. However, voters in several states complained of long lineups, while others encountered technical glitches, including one voting machine in Pennsylvania that indicated a vote had been cast for Romney when the Obama button was pressed.

In other precincts, voters complained of robocalls giving them false voting information.

As polls began closing on the East Coast, Romney took a handful of states, including Kentucky and its eight electoral votes, followed by West Virginia, Indiana and South Carolina.

Obama was quickly declared the winner in Vermont, taking its three electoral votes, and sat there until nine states came through for him, including his home state of Illinois and its 20 electoral votes.

Unlike Obama, Romney lost his home state, with Massachusetts鈥� 11 electoral votes going to the president.

In the U.S., the popular vote does not decide who wins the election. Rather, the winner of each state gets that state鈥檚 Electoral College votes, which are assigned based on that state鈥檚 representation in Congress. The winner needs 270 electoral votes.

The popular vote is the key indicator of voter intentions, and Romney had a slim edge throughout the night at 51 per cent.

When it comes to the issue of most concern to voters, a national exit poll found most said the economy is the top issue facing the nation.

Indeed, the winner will be tasked with turning around a sluggish economy and reining in a national debt that tops $16 trillion and a budget deficit that has reached $1 trillion.

Tuesday鈥檚 exit poll of more than 19,000 voters conducted by Edison Research for The Associated Press and the major U.S. networks found that:

  • 59 per cent of voters chose the economy as the biggest issue facing the country.
  • Just under four in 10 voters said unemployment was the biggest economic problem they are facing.
  • Four in 10 voters said the economy is improving, while 3 in 10 said it is getting worse.

Obama wound down his campaign late Monday, and spent election day in Chicago, where he visited volunteers at a campaign office and worked the phones to get voters to the polls.

Obama, who cast his ballot last month in early voting, congratulated Romney on a 鈥渟pirited campaign鈥� and said that while he feels he has the votes to win, 鈥渋t鈥檚 going to depend ultimately on whether those votes turn out.鈥�

Obama also played basketball with friends while he waited for polls to close.

Romney voted with his wife, Ann, early Tuesday morning near his home in Massachusetts before meeting running mate Paul Ryan in Cleveland, Ohio, for some last-minute campaigning. Romney also attended a rally later Tuesday in Pennsylvania.

Vice President Joe Biden also made a last-minute, unannounced stop in Cleveland, as the two camps reached out to voters in the swing state that has chosen the last 12 presidential election winners.

Polls had Obama and Romney in virtual ties heading to voting day. A Washington Post-ABC News poll released Monday had Obama at 50 per cent support, with Romney at 47 per cent.

CTV鈥檚 Washington Bureau Chief Paul Workman said Tuesday that Obama did not inspire the same level of voter enthusiasm as he did in 2008, when he ran on a campaign of hope and change.

Workman said Obama may have benefited from the arrival of Hurricane Sandy on the shores of the U.S. Northeast, his handling of which won him high praise.

鈥淗e was all of a sudden the president again, making very presidential decisions, the focus was back on him,鈥� Workman told CTV鈥檚 Power Play from Chicago. 鈥淏ut also what it did was it stopped whatever momentum the Mitt Romney campaign had going for it, stopped it dead for three or four days. So that may very well have helped Obama.鈥�

CTV鈥檚 Joy Malbon said the mood in the Romney party headquarters in Boston was 鈥渃autiously confident,鈥� where supporters hoped his strong performance in the first debate won over enough undecided voters.

鈥淭hey鈥檙e not going to admit they鈥檙e losing, but certainly their road to victory is much harder than the president鈥檚,鈥� Malbon told Power Play.

In addition to voting for a new president, Americans in some states were voting on local measures on issues such as gay marriage and the legalization of marijuana. All 435 seats in the House of Representatives are up for grabs, as well as one-third of the Senate鈥檚 100 seats.

Governors are also being chosen in 11 states.

With files from The Associated Press