For months, South Dakota Democrats have suggested that Native American voters might provide just enough votes to help Sen. Tim Johnson win reelection over GOP Rep. John Thune. But an emerging scandal over the Democratic Party's outreach to tribes is roiling the already fiercely contested campaign.

State and federal authorities have begun a criminal investigation into whether the Democratic Party's assiduous courting of Native Americans produced fraudulent voting registrations and absentee ballots. Four hundred registration cards and absentee ballot applications are under scrutiny for possible irregularities such as forged signatures and dead registrants.

Republicans have even hinted at a legal challenge to the Nov. 5 election results. Democrats say the GOP simply wants to suppress Native American turnout in a pivotal election.

No one questions that Democrats have worked diligently to register Native American voters after pledging to increase the total to 20,000 from 10,000 at the campaign season's start. About 17,000 new South Dakota voters had registered by Monday's deadline, state officials said. About one-fourth live on or near Indian reservations.

South Dakota Democratic Party spokeswoman Sarah Feinberg said the party is extremely proud of its outreach efforts, having established offices on every reservation. "It's extremely important to us that every eligible and legitimate voter have an opportunity to register and participate in the political process," she said.

Native Americans tend to vote Democratic, a critical factor in the sparsely populated state. Johnson won his last race by fewer than 6,000 votes, and this year's face-off with Thune -- as well as the House battle between Gov. Bill Janklow (R) and Democratic lawyer Stephanie Herseth -- could be decided by even smaller margins.

In recent weeks, evidence of tainted registration cards and ballot applications has clouded the Democrats' efforts. Early this month, party officials determined that at least two absentee ballot applications were fraudulent. They fired a contractor, Becky Red Earth-Villeda, over allegations she had submitted scores of illegal voter registrations. One came from a woman who had died more than two weeks before officials received the card, and many more had mismatched birth dates and nonexistent addresses.

The FBI is working with state and local officials to determine how many registration cards and ballot applications may be tainted. On Tuesday a consultant for the Sioux Tribes Voter Education and Registration Committee, Lyle Duane Nichols, was indicted on five counts of forgery in connection with voter registration cards he submitted.

"It's vitally important the public know when these kinds of allegations are made that they are going to be investigated and investigated quickly," said U.S. Attorney James McMahon.

Republicans are using the controversy as a political issue. Thune and his supporters have made much of the fact that this spring Johnson said his campaign was "trying to secure space to set up campaign offices on each reservation in the state." Johnson spokesman Dan Pfeiffer said the campaign ultimately let the state party establish the offices in question, where officials are "working to elect Tim Johnson and every other Democratic candidate on the ballot this fall."

In a debate Monday night, Johnson challenged Thune to halt all negative campaigning. Thune said he would do so as soon as Johnson held a news conference explaining what he knows about voter fraud in the state.

Charges and counter-charges keep flying. Democrats question why the state GOP paid for a lawyer to travel to San Antonio to study election fraud, and suggest the lawyer leaked word of voting irregularities. They say only 130 documents under scrutiny are actually tied to the Democratic Party.

"It is clear that John Thune has engaged in a clear and concerted effort to discredit this registration drive and to suppress turnout on the reservations," Pfeiffer said.

Republicans say their lawyer's election fraud training was routine. They note that Democrats, not Republicans, were the ones paying people to collect registration cards and absentee ballot applications on reservations.

Thune spokeswoman Christine Iverson said the Johnson campaign has "organized a massive voter registration effort on the reservations. They are going to attempt to win this election on the reservations. Now that the FBI is investigating their efforts on the reservations they are attempting to deny all involvement. They can't have it both ways."

It's unclear how the controversy might affect voters' decision, but even a little impact could prove decisive.

"In an election where they think 5,000 votes may separate the winner from the loser in the Senate race, this is an important issue," said William D. Richardson, head of the University of South Dakota's political science department.

Republicans have attempted to link Sen. Tim Johnson (D-S.D.) to allegations of irregularities in voter registration efforts on Native American reservations.