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
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