Republican challenger Tom Kean accused Democratic incumbent Robert Menendez of "Election Day dirty tricks" in New Jersey's hard-fought Senate race.
"Late last night vandals struck the Kean for Senate headquarters ... by chaining closed the main entrance to the building as well as breaking off keys in the side door entrances," Mr Kean's campaign manager Evan Kozlow said in a statement which slammed the stunt as a "desperate ploy" by Mr Menendez's operatives.
Republican officials also allege voting machine rigging in at least seven polling places throughout New Jersey.
"We have received dozens of calls from predominately Democrat areas throughout the state complaining of voting machines being preset with votes for Menendez, poll worker misconduct, and machines not registering votes for Tom Kean per the voter's intent," said Mark Sheridan, New Jersey Republican State Counsel, in a statement.
"Upon entering the voting booth, individuals have encountered machines that are pre-set to vote for Menendez, causing serious confusion. In some instances, the voter has been unable to de-select the pre-set Menendez vote using the touch screen. A number of voters also have called to express concern that they mistakenly voted for Menendez," he said.
"Given the frequency of these issues in what are traditionally Democrat areas, we are deeply troubled by these complaints coming from New Jersey voters."
Officials representing Republican challenger Mike Bouchard in Michigan alleged that his Senate campaign had been disabled by a cyber-attack, as he attempts to unseat Democratic incumbent Debbie Stabenow.
FBI Investigation
Meanwhile in Virginia, FBI agents are investigating allegations of voter intimidation lodged by the Democrats.
Federal Bureau of Investigation agents spoke with the top elections officials in Virginia about voter abuse cases, said Dee Rybiski from the FBI's Richmond office.
Ms Rybiski gave no further details, but the Virginia Democratic Party said that voters over the past days "have filed complaints of incidents aimed at suppressing voter turnout in heavily Democratic and African American neighbourhoods."
In one transcript of an alleged call, a Democrat named Tim Daly received a call claiming to be from the "Virginia Elections Commission" to say that Daly is registered to vote in New York.
"Therefore, you will not be allowed to cast your vote on Tuesday. If you do show up, you will be charged criminally," the message says, according to a transcript posted on the website of Democratic Senate candidate Jim Webb.
Another Virginia voter said he received a call from someone claiming to be in the Webb campaign to say his polling station had moved -- even though Cox has been voting at the same site since 1973.
The call was traced to the western state of Montana, and similar calls were received from California, according to the Democrats.