WASHINGTON - Hundreds of South Jersey Democrats' votes in the most hotly contested congressional district in the region could be invalidated after Republicans challenged a swath of mail-in ballots.

In a filing Friday, the Burlington County Republican committee challenged the ballots because the Democratic state committee left out required information when sending mail ballot applications to their supporters in a U.S. House race between the GOP's Tom MacArthur and Democrat Aimee Belgard.

Democrats acknowledge that they failed to fill out the "assistor" section showing they had helped voters.

But they called it a "technical oversight" and cast the GOP's challenge as an attempt to disenfranchise their supporters. Republicans said Democrats should know the rules and follow them.

"They would challenge us if we hadn't provided assistors," said Burlington County Republican chairman Bill Layton. "For them to say this is anything less than us protecting the integrity of the election is malarkey."

Belgard's campaign manager, Hannah Ledford, said MacArthur should "call off the dogs" and demand that the GOP drop its challenge. "The Republican party's attempt to rob Americans of their right to vote is shameful," Ledford said in a statement.

Some 600 ballots were being scrutinized when the GOP filed its challenge Friday afternoon, said Evan Crook, a lawyer for the Burlington County Republicans. The number could rise.

The ballots in question covered not just the congressional race, but the U.S. Senate contest and local elections in a district that includes much of Burlington and Ocean Counties. It has 491,000 registered voters.

MacArthur, a former mayor from North Jersey, and Belgard, a Burlington County freeholder, are running to replace the retiring Republican Rep. Jon Runyan in a taut race that has drawn national attention.

"We believe that every appropriately cast vote should be counted," MacArthur spokesman Chris Russell wrote.

The decision of whether to count the ballots is up to the Burlington County Board of Elections, a bipartisan panel.

The dispute stems from a mailing the Democratic state committee sent to registered Democrats and other likely Belgard voters in October. They received mail-in ballot applications - with the required voter information already filled in.

"Being a voter is as easy as 1-2-3," read the mailing.

If their applications were accepted, voters received a ballot and could vote by filling it out and mailing it in - an easy method campaigns often encourage. New Jersey allows mail-in voting for any reason.

But Democrats did not fill in the "assistor" section of the applications, required when a voter gets help. The forms ask for an assistor's name, signature, and mailing address.

In 2005, New Jersey's then-attorney general, Peter C. Harvey, a Democrat, addressed a similar issue surrounding the "assistor" section on absentee ballots, Crook said. The opinion, he said, made clear that the section must be filled out.

"For Democrats to suggest that there's any type of voter suppression, they know better," Layton said. "This was their attorney general."

Democrats tried to address the issue by having the person who supervised the mailing, Maren Hesla, fill out a sworn affidavit attesting that she was the "assistor."

Hesla works for a political mail firm, Mission Control, in Connecticut. Her affidavit was notarized Thursday, a day before the GOP challenge.

Republicans said the affidavit did not suffice. "It was submitted after the fact," Crook said.