WASHINGTON -- Provisional ballots, one of the fixes the government implemented following the disputed 2000 election, are often proving to be a poor substitute for the real thing.

A 2002 law, the Help America Vote Act, requires states to issue provisional ballots to voters whose eligibility is uncertain when they arrive at the polling station. But in an election system that gives state and county officials wide discretion in managing elections, voters get varying treatment depending on where they live.

Some states rarely issue provisional ballots. Others do it frequently, sometimes even over minor differences between a voter's identification and data on the rolls. Some states end up counting only a minority of the provisional ballots. In other states, provisional voters have better odds.

The issue matters because the legal battles being fought by Democrats and Republicans over registration issues may result in more voters using provisional ballots in key states such as Ohio and Florida. The partisan legal stances being established now set the stage for possible fights over which ballots get counted.

In 2004, 1.9 million provisional ballots were cast nationwide, of which 676,000 weren't counted. In the 2006 election, 20% of the 800,000 provisional ballots were discarded.

The ballots are supposed to ensure that no voter is turned away at the polls. But the 2002 law leaves it up to states to devise their own rules for determining which ballots are valid.

In this year's primaries, the rejection rates varied widely by state, according to data collected by the Pew Center on the States. In Illinois, only 4,447 of the 15,205 provisional ballots cast counted. In Maryland, 31,781 of the 54,038 provisional ballots were deemed valid. In Kentucky, with nearly 900,000 voters casting ballots, only 102 provisional ballots were issued and only eight counted.

A study focusing on Florida and Ohio that was released last month by the Advancement Project, a civil-rights group, said a majority of the provisional ballots being discarded are cast by voters who are otherwise eligible. They get thrown out because voters show up at the wrong precinct and don't get help finding the right polling station, or because voters get confused by extra paperwork such as a requirement to sign the ballot.

"Provisional ballots are often used improperly," says Jon Greenbaum, director of the voting-rights project for the Lawyers' Committee for Civil Rights, which works for minority rights. He says there is cause for concern "when you have eligible voters whose votes aren't counted because of a technicality."

Others say there isn't enough evidence to show whether a high rate of discarded provisional ballots is bad for voters. The rejection rates may be a sign that systems are stopping ineligible voters from casting ballots. They say it is up to voters to get to the right polling place and ensure their registrations are up to date and error-free.

Still, there is little data to explain why states vary so widely in the number and treatment of provisional ballots. Instructions to poll workers may explain part of the divergence. In some places, "a provisional ballot becomes a catch-all escape hatch for poll workers who are dealing with a thorny issue," says Doug Chapin, election expert at the Pew Center on the States.

The problem could get messy if races are tight in places such as Ohio and Florida, as partisan officials try to sway the rules to favor their side. Local election boards often rely on a mix of volunteer and paid poll workers whose level of training varies and whose errors can have a big impact on whether voters can cast ballots. Registrations are often handled by clerks whose typing mistakes can end up forcing a voter to cast a provisional ballot.

Already some provisional ballots are being reported in some states where early voting has begun. In Broward County, Fla., a voter reported being issued a provisional ballot because she had moved from neighboring Palm Beach County and there was a mismatch between registration rolls and her identification.

The voter made her complaint to an Election Protection telephone hotline run by a consortium of civil-liberties and voting-rights groups, including the Electronic Frontier Foundation.

Broward County elections officials said they couldn't verify the complaint, but said in a statement that based on the voter's description of events she "was issued the correct ballot style, based on the new Broward address, [and] the ballot will be accepted when reviewed by the Canvassing Board on November 4."

Write to Evan Perez at evan.perez@wsj.com