HONOLULU -- Hawaii voters for the first time must pick a political party when voting in this year's primary election, a requirement that election officials hope will result in fewer ballots being thrown out.

But members of both the Democratic and Republican parties worry that voters could get confused during the Sept. 20 primary, resulting in their votes not being tallied correctly.

The primary could decide mayoral races on Oahu, the Big Island and Kauai and will pick final candidates in legislative races.

Voters in Hawaii, as in most states, have always only voted for one party's candidates in primary elections, which are used to narrow each party's candidates to one per race before the Nov. 4 general election.

The 2008 election is different because voters will have to choose a political party before filling out the rest of the ballot. In previous years, voters were handed ballots color-coded by party; this year, everyone fills out the same white ballot.

State Election Chief Kevin Cronin said the new voting machines will record votes from the selected political party and disregard stray votes, which previously could have resulted in the whole ballot being thrown out.

"More votes will be counted this year," Cronin said. "It could dramatically increase the accuracy of the ballots."

In 2006, 5,231 ballots were invalidated for multiparty voting during the primary, accounting for nearly 2 percent of votes cast.

Hawaii is using paper scan and electronic voting machines made by Hart InterCivic this year under a $43 million contract intended to run through 2016. A ruling by a state administrative hearings officer has ordered that the contract be rebid, finding that the cost was "clearly unreasonable" compared to an $18 million bid from rival company Election Systems & Software. Hart filed an appeal in Circuit Court on Monday.

Hawaii's political parties argue that some voters may accidentally select the "Independent Party" with the belief that they're voting as independents. A voter who chooses to vote in the Independent Party primary must vote solely for Independent Party candidates in partisan races. Neither of two Independent Party candidates have opponents in the primary election.

"What we're concerned about is the chance that someone might half-consciously check the Independent box and then vote the straight Democratic Party ballot and then have their votes voided," said Bart Dame, a Democratic Party elections observer who has seen the ballot.

The Elections Office acknowledges that voters could make that mistake, but they believe the instructions on the ballot will guide voters through the process. In addition, voting machines will return ballots with overvotes, and voters will be allowed to redo their selections before they leave the polling place.

"In America, most people believe they are independent, with a small i," said Willes Lee, chairman for the Hawaii Republican Party. "That is going to confuse some people and cause a lot of votes to be lost."

But the greatest potential for lost votes comes from people who turn in absentee ballots, which made up more than a third of total votes cast in the 2006 Hawaii primary election.

Unlike at a polling place, an absentee ballot can't be corrected if a voter fills it out incorrectly. Write-in votes are not allowed in Hawaii elections.