Henry, 43, was bidding to become only the third Oklahoma governor to win re-election.
Mike Clingman, state Election Board secretary, predicted a moderate turnout of about 900,000 voters, about 44 percent of the electorate. That would be a significant improvement in the 24 percent that turned out in the July primary and the 17 percent that voted in the runoff in August.
Henry is running on his record, which includes signing a $627 million tax cut this year, while giving teachers a $3,000 pay raise and increasing funding for highways, health care and other popular programs.
“I've done what I said I would do. I've worked well with everyone, regardless of party, and we've accomplished a lot in four years,” Henry told The Associated Press at the start of the campaign.
Both Henry and Istook, 56, are lawyers. Henry is a former state senator and Istook was a state House member before being elected 5th District congressman in 1992. He was re-elected six times.
Public opinion polls have consistently showed Henry leading Istook by more than 20 points and he had more than a 3-to-1 advantage in fundraising.
Istook supporters said the congressman made up ground by putting Henry on the defensive in recent television debates on such issues as crime and illegal immigration.
The congressman blasted Henry for paroling too many drug dealers. Henry said the state's parole rate was less than the national average and less than states in the region.
Istook got a bit of luck last week when he ran a television ad showing Henry's announcement delivering Oklahoma Democrats' votes to Massachusetts Sen. John Kerry at the Democratic National Convention in 2004.
About the same time, Kerry made a much publicized remark interpreted as questioning the educational qualifications of U.S. troops in Iraq.
Kerry, who is considered too liberal for conservative Oklahoma, had to apologize and stop campaigning in states with hotly contested congressional elections.
During the candidates' fourth and final debate on Thursday, Henry strongly condemned Kerry's remarks about soldiers in Iraq, but said he liked Kerry's domestic ideas.
“That tells us a lot, governor,” Istook said, referring to Kerry as the second most liberal members of the Senate next to Sen. Ted Kennedy.
It appeared that Istook's lack of financing, however, would prevent him from fully exploiting what his supporters said was a misstep by Henry.
Derek Kern, Istook's press secretary, said it was too late to get a new TV ad out on the Kerry matter and the congressman would be unable to match the frequency of ads Henry will run in the final days before the election.
Paul Sund, a spokesman for Henry, said Istook had a small TV effort, compared to gubernatorial candidates in previous elections.
Istook's campaign was hampered early on by bad publicity over his having to return contributions from disgraced lobbyist Jack Abramoff and Abramoff's clients.
He also was roughed up in the primary by an opponent who ran a TV ad in which a “Hee-Haw” comic donned a pig mask to illustrate an assertion that Istook was part of the spending problem in Washington.
Istook prides himself on being a fiscal and social conservative. He made his mark in Congress by pushing for prayer in school and a budget-balancing amendment.
Istook also seemed to be searching in vain early in the campaign for issues to use against the popular Henry, who said his bipartisan approach to governing contrasted with “the Washington way of sniping and partisan bickering and getting no results.”
Istook initially criticized Henry over the lottery and the tobacco tax increase. Henry replied that those two issues were overwhelmingly approved by voters to raise money for education and health care.
In the case of the lottery, Henry said he was fulfilling a campaign promise to put the issue to a public vote.
Istook finally seemed to strike a chord with some voters by charging the Henry had not done enough to fight illegal immigration. Among other things, he criticized the incumbent for signing a bill that allowed illegals to get tuition aid at state colleges and universities.
Henry replied that illegal immigration was mainly a federal problem that Istook and other members of Congress had failed to resolve. He said he would not have signed the tuition aid bill if it did not contain a provision requiring those getting the aid to be working toward legal status.
The two candidates also differed over the Oklahoma economy. Henry said the state was on the move, producing jobs and record state revenues. He said he had used the extra cash wisely to build up the state's Rainy Day Fund, make tax cuts and invest in education, highways and other important programs.
Istook said the economy was mainly bolstered by high oil and natural gas prices and Henry was “lucky” to be governor at this time. He said the state was losing high-paying jobs and he would work to bring them back.
Henry is seeking to join Democrat George Nigh and Republican Frank Keating as the only governors elected to successive terms.