US vice-president Joe Biden has criticised Republican candidate Mitt Romney for proposing policies the Democrat claims would cut taxes for millionaires and raise them for middle-class families.
By comparison, Mr Biden said President Barack Obama's plan asks the wealthy to pay only a little bit more in taxes.
Mr Romney's campaign has said claims by the Obama camp about Republican tax plans are false.
Mr Biden told about 1,000 supporters in Charlotte, North Carolina, that he does not understand how Mr Romney can justify raising taxes on the middle class.
Separately, Mr Biden said he expects Mr Obama to be re-elected because he will win North Carolina for a second time.
Meanwhile, Mr Romney's campaign has sent letters to election officials in Wisconsin, Mississippi and Vermont demanding that the deadline for receiving ballots from military and overseas voters be extended.
The letters sent last week and yesterday on behalf of the Romney campaign by former US Veterans Affairs Secretary Anthony Principi say election officials in each of the states missed the September 22 deadline for posting some ballots to overseas and military voters.
Romney campaign spokesman Ryan Williams said a fourth letter was to be sent to Michigan officials.
A spokesman for Wisconsin's elections board said 44 ballots were mailed to military and overseas voters within five days after the deadline.
Mr Romney's push is part of a broader effort to court the veterans vote.