Warning over US election problems
Some early voters in Florida and North Carolina faced a long wait
Researchers are warning of potential problems during the US election with record numbers set to vote and many states using new voting machines.
Long queues are likely at polling stations on 4 November, Pew researchers say, and both parties are hiring lawyers in anticipation of challenges.
Voters have already had long waits in some states where early voting is under way, like North Carolina and Florida.
It comes despite efforts to improve the system after problems in 2000 and 2004.
The 2008 election "has the potential to combine a record turnout with an insufficient number of poll workers and a voting system still in flux," the report by the non-partisan Pew group says.
The biggest hurdle facing election workers may be the new voters registering in record numbers in almost every state, the report says.
Millions of new voters have registered across the US in the run-up to the vote
For example, officials in Virginia recently ordered 200,000 extra voter registration forms.
And although many states are encouraging people to cast their ballot early or send it in by post, there is still a danger of big queues on election day and insufficient numbers of poll workers to handle the influx, the report warns.
Election officials in Virginia have said they will step up polling station security amid concerns that arguments over long queues, voter registration and identity issues could become heated.
Analysts suggest that early voting in a number of key states is favouring Democratic presidential candidate Barack Obama.
In North Carolina, some 214,000 people cast their ballot on the first two days of early voting, with registered Democrats making up 62% of the number compared with 22% registered Republicans.
Meanwhile, a new opinion poll by the Pew Research Center suggests Mr Obama has increased his national lead over rival John McCain in the past month to 14 points, with 52% to his 38%.
The latest NBC/Wall Street Journal poll gives Mr Obama the same lead over Mr McCain, up from a six-point margin in the same poll two weeks ago.
Mr Obama and his running mate, Senator Joe Biden, are to meet his campaign's national security advisers in Virginia on Wednesday.
After the discussion, Mr Obama is expected to give a public briefing on how his foreign policy plans compare to those of his rival.
McCain continues to talk about Obama 'spreading the wealth', which I understand as a political tactic might be effective
It comes a day after Mr McCain questioned his rival's readiness for the White House, as he campaigned in Pennsylvania.
"We don't want a president who invites testing from the world at a time when our economy is in crisis and Americans are already fighting in two wars," Mr McCain said.
Mr McCain was expected to return on Wednesday to New Hampshire, a state he won in the Republican primaries but which opinion polls suggest is leaning towards his rival.
Mr Obama will also hold rallies in Richmond and Leesburg, Virginia, on Wednesday in which he will focus on the economy.
He will campaign in Indiana - another traditionally Republican-leaning state where he is doing well in the polls - on Thursday, before taking a two-day break to visit his sick grandmother in Hawaii.
Clashes on tax
On Tuesday, the second day of a swing through Florida, Mr Obama accused Mr McCain of making "stuff" up in the last weeks of the campaign.
Barack Obama blames "irresponsibility in Washington" for financial turmoil in the US
Earlier, Mr Obama met the governors of Ohio, Michigan, New Mexico and Colorado in Lake Worth, Florida, to discuss jobs and the economy with business leaders and financial experts.
On the campaign trail in Pennsylvania, where Mr Obama has the lead in opinion polls, Mr McCain said his rival's economic plan would result in raised taxes.
Meanwhile, his running mate Sarah Palin apologised for any misunderstanding over comments last week on the patriotic values of "the real America" and "pro-America areas of this great nation".
Mrs Palin denied that was her intention to imply that some parts of the country were more patriotic than others.
"I don't want that misunderstood. If that's the way it came across, I apologise," she told CNN.