Japanese voted Sunday in an election expected to strengthen Prime Minister Shinzo Abe's hand, giving him the power to push through economic reforms and bringing the country political stability.
Half of the 242 seats in the upper house of parliament are up for grabs.
With approval ratings averaging well over 60 percent since he won the premiership in a general election last December, victory for the hawkish premier's Liberal Democratic Party (LDP)and its allies is all but assured.
Apart from a brief interval under the previous government, it would be the first time for six years that a ruling party will control both houses of parliament.
The legislative deadlock of recent years created a succession of "revolving door" prime ministers.
After Abe resigned his previous term as prime minister in 2007 following a defeat in upper house elections, Japan had five prime ministers until his comeback last year.
Sunday's predicted swing towards Abe's LDP and its junior coalition allies could give Japan its first long-term prime minister since Junichiro Koizumi, who ruled for the first half of the last decade.
"This election is a fight to achieve political stability," Abe's LDP said in a statement. "By winning the fight, our party can assume our responsibility to the nation."
"The Abe administration can become a long-run regime," said Koji Nakakita, professor of politics at Hitotsubashi University in Tokyo.
"It even has the potential to go beyond the next three years."
Polling stations close at 8:00 pm (1100 GMT), with broadcasters' exit polls expected shortly thereafter.
Voter turnout was 22.66 percent as of 2:00 pm, down 5.15 percentage points from the last upper house poll in 2010.
Abe's supporters say a win will give him free rein to force unpopular changes on the world's third largest economy, which suffered deflation for more than a decade but grew at an annualised rate of 4.1 percent in the quarter to March.
They point to the need for reform of the labour market to make it easier for firms to hire and fire workers, call for participation in a huge free trade pact and seek a rise in the consumption tax to curb the increase in the sky-high national debt.
The planned reforms are the third instalment of an economics policy dubbed "Abenomics" that has already seen a slew of government spending and a flood of easy money from the central bank.
Those moves pushed down the value of the yen to help exporters and sent the stock market soaring.
"We are on the threshold of economic recovery," Abe told voters on the eve of Sunday's voting.
"This election is virtually a referendum on Abenomics and a gauge to measure voters' expectations from it," said Yoshinobu Yamamoto, professor of politics at the University of Niigata Prefecture.
A strong public mandate would allow Abe to face down opposition within his own party to the planned free trade pact, which is opposed by farmers and the many legislators from rural constituencies, and to other reforms.
Abe's detractors fear he will return to his conservative social agenda once he has control over both houses of parliament.
They fear this will mean a loosening of Japan's constitutional commitment to pacifism, a boosting of the military and a more strident tone in already-strained relations with China and South Korea, both of whom have territorial disputes with Tokyo.
But many voters voiced hope for economic recovery under Abe's leadership, while others lamented the weakened opposition.
"I hope the Abe cabinet will exercise strong leadership to push for recovery so that we can actually share and feel it," said Masako Suzuki, a 82-year-old pensioner, at a Tokyo polling station.
Miho Sugiyama, a 35-year-old Internet firm executive, said: "I can't vote for any opposition parties, which all lack ability to take action. I don't expect much from the current opposition."
The largest opposition Democratic Party of Japan, which lost last year's lower house election, is expected to suffer another blow in Sunday's poll, according to surveys.