Media surveys show Abe's Liberal Democratic Party (LDP) and its junior coalition partner on track to win more than half the 124 seats up for grabs in the election, possibly strengthening their majority in the chamber.
Up in the air, however, is whether the ruling bloc and its allies will keep the two-thirds "super majority" needed to begin the process of revising the constitution's pacifist Article 9 to further legitimise the military, a controversial step.
"If they lost it (the two-thirds majority), constitutional revision would be impossible," said Steven Reed, an emeritus professor at Chuo University.
The charter has never been amended since it was enacted in 1947 and changing it would be hugely symbolic, underscoring a shift away from post-war pacifism already under way.
Article 9, if taken literally, bans maintenance of a military but has been stretched to allow armed forces for self-defence. Surveys show voters are divided over changing it, with opponents worried doing so would increase the risk of Japan getting entangled in US-led conflicts.
STABILITY FIRST?
Abe, who took office in December 2012 pledging to restart the economy and bolster defence, is pushing his LDP-led coalition as the best bet for political stability.
Opposition parties have focused on what they call a threat to voter finances, including a potential hit on spending from an October rise in the sales tax to 10pc and strains in the public pension system in the shrinking, fast-ageing population.
Voter Yu Suzuki, 30, in Niigata, northern Japan, said she was worried about pensions but would vote LDP. "Compared to other parties, the LDP feels more secure," she said.
The ruling bloc along with the Japan Innovation Party and independents open to constitutional revision need to win 85 seats to keep a two-thirds majority, media calculations show.
Voter interest in the poll has been tepid and turnout could fall below the 54.7pc in the last upper house poll in 2016.
"We cannot change our society just by complaining, and I believe young people who have complaints should raise their voices," said 37-year-old Tokyo voter Junichi Nakada.
"Although I'm not young any more, I came to vote today to raise voter turnout."
Voting ends at 8 pm (1100 GMT) with media likely to call the outcome late at night or early on Monday morning. Official results are not expected until Monday.
Abe has led his party to victory in five national elections since returning as LDP leader in 2012, and is on track to become Japan's longest-serving premier if he stays in office until November. But the victories have been aided by a fragmented opposition and low turnout.
The main opposition Constitutional Democratic Party of Japan was expected to increase its seats but remain dwarfed by the LDP.
Some voters said domestic issues were only one part of the picture.
"I hope that the next ruling party and leaders will be able to take a solid look not only at Japan's domestic issues but also how Japan works with neighbouring countries," said Noriko Yasuhara, 63.
"Or with regards to relations with the United States, what Japan's role will be."