Case, 53, faced the difficult challenge of unseating the 82-year-old Akaka, who led in pre-election polls and who has cited his consistent votes against the Iraq war and his 30 years of Congressional experience as reasons voters should back him.
Democratic-leaning Hawaii has never unseated an incumbent member of Congress since it became a state in 1959, but Case has centered his campaign around the idea that voters need to boot out Akaka in favor of a more youthful representative who can start to build up seniority.
Polls close at midnight EDT, with the first results from the day's voting to be released at 3:30 a.m. EDT Sunday. But a record number of absentee and early votes were to be released when the polls close, giving an early indicator.
Two recent polls had showed Akaka with a substantial lead among likely Democratic voters, but many residents were undecided and Case could benefit from Republicans and independents who take a Democratic ballot in the open primary.
Akaka, who drew endorsements from leading Senate Democrats, held more than a 2-to-1 advantage in fundraising, collecting more than $2 million to Case's $817,000 by early this month.
Hawaii has the lowest voter turnout in the nation, with even less participation in primaries. Only 40 percent of 626,000 registered voters cast ballots in the 2004 primary.
Nearly 95,000 people had voted by absentee ballot and walk-in voting as of Thursday — more than one-third of the total number of people who voted in the primary two years ago.
The winner faces no serious GOP opposition in the general election, although Republican Gov. Linda Lingle asked voters to nominate motivational speaker Jerry Coffee, who pulled out of the race because of illness. That would enable the party to pick a new candidate for November.
The race for Case's congressional seat developed into a free-for-all, with a dozen experienced candidates seeking the nomination. The primary winner is favored to win the general election, keeping Hawaii's congressional delegation all Democratic.
Two little-known Democrats are competing to take on Lingle in November, but she is expected to easily win a second term regardless of her challenger. Political newcomer William Aila, a harbormaster, is hoping to upset former state Sen. Randy Iwase.
Election officials planned to use a new form of electronic voting that will leave a paper trail to help verify votes.