PORTLAND, Ore. — The tight race between Sens. Hillary Clinton and Barack Obama has attracted national attention not only to today's Oregon primary but also to the state's unique vote-by-mail system.

It's a key test for mail balloting. More than a dozen states are expanding vote-by-mail or considering expansion, but Oregon remains the only state with 100 percent mail balloting, after voters approved the change a decade ago.

With so much at stake, the last thing Secretary of State Bill Bradbury wants is an election day snafu. That's why he was "hugely concerned" after the state mailed 33,500 Oregonians two ballots apiece for today's election.

Those voters changed their party registration close to the April 29 deadline, after election officials had prepared ballots for mailing. The voters each received a ballot based on their old information and one with their updated registration.

"There was a real possibility that the whole thing would look completely fraudulent because of a bunch of people getting two ballots," Bradbury says. "You just really don't want that image."

But Bradbury and other election officials say the vote-by-mail system has enough safeguards in place to assure that each voter will vote only once. They sent notices and postcards to each of the voters advising them what to do, and now they are confident there won't be double voting.

"I have no question that the signature verification process will catch virtually every one of these," Bradbury says.

The Clinton and Obama campaigns are watching closely. Spokesmen for both campaigns say they aren't aware of problems and expect everyone to vote as intended  — once.

"However, we'll wait and see how it goes," said Obama spokesman Nick Shapiro.

Under vote-by-mail, there are no polling places. Voters get their ballots in the mail and have more than two weeks to send them back or deliver them to a drop-off site.

Oregonians clearly prefer voting by mail, approving it in 1998 with 69 percent of the vote. It has other supporters as well. The Vote by Mail Project, based in Portland, points to benefits such as increased voter participation and convenience.

Turnout for the 2004 primary was 46 percent in Oregon. Bradbury's office expects even higher turnout Tuesday, as much as 60 percent.

There also are detractors. Some voters still prefer traditional polling places. The No Vote by Mail Project, based in Washington state, includes a long list of concerns such as fraud, ballot secrecy, security and increased campaign costs due to the longer period that ballots are in the hands of voters.

Critics note that Oregon didn't have thousands of voters getting two ballots before voting by mail.

"It's a problem that wouldn't exist if you had polling places," notes Dan Lavey, a partner in Gallatin Public Affairs, a consulting firm.

Voters getting two ballots could lead to two problems. One is that voters could wind up voting twice. The other is that voters will be confused about what they are supposed to do, resulting in their not voting or voting differently from how they intended to.

John Lindback, Bradbury's director of elections, insists that those voters will not be able to cast two ballots in this election.

Each ballot carries unique coding on the envelope. When the second, updated ballot was sent to some Oregon voters, the coding for their first ballot was canceled. If that envelope is returned to election officials, the computer system will catch it when the bar coding on the envelope is scanned.

"The more serious problem is confused voters, rather than a systemic problem where two ballots are going to get counted," Lindback says.

Lindback and others said all Oregon elections have some voters receiving two mail ballots. What's different is that there were so many this time.

The high number resulted from a combination of the increased interest in the Clinton-Obama race and the timeline used for Oregon's mail elections.

The registration deadline was April 29, just three days before ballots were mailed. But counties had started preparing ballots as much as two weeks earlier. So when thousands switched their registration close to the deadline, they did so after a ballot had already been loaded with others in boxes, ready for mailing.

One possible fix: Set an earlier deadline for changing voter registration.

"If you moved that back a full two weeks for party switching, you would essentially solve that problem," Lindback says.

He says he expects the 2009 Legislature to at least discuss the two-ballots problem when it convenes in January.

John Fortier, a research fellow at the conservative American Enterprise Institute in Washington, D.C., authored the 2006 book "Absentee and Early Voting: Trends, Promises and Perils." Though he is generally critical of absentee voting, his impression is that Oregon officials "take a lot of care in trying to do things right."

Whether it could lead to a legal challenge in Oregon will depend on whether problems develop affecting a close race.

In a tight race, every potentially troublesome ballot gets examined, notes Loyola Law School professor Richard Hasen, who writes an election law blog on the Internet.

"If it's true that state officials ensure that no voters get to vote twice, then there's no problem," Hasen says. "But if somebody makes a mistake and somebody gets to vote twice, then it could be a problem."

Bradbury said he doesn't expect voters getting two ballots to slow the spread of vote-by-mail.

Under Oregon's law, ballots must have been received by county election officials by 8 p.m. PDT today. Initial results are released at that time and updated until all ballots have been counted.

(Dave Hogan is a staff writer for The Oregonian of Portland, Ore. He can be contacted at davehogan(at)news.oregonian.com.)