Prime minister Fumio Kishida's ruling party - which has governed Japan for 62 of the past 67 years - was supposed to win over 70 of 125 seats in the upper house alongside right-wing allies Komeito.
Though a change in government through the more powerful lower house was not at stake, today's elections are supposed to be a referendum on Japan's leaders - and the legacy of its slain ex-PM.
The estimated results, reported by Japanese state broadcaster NHK, would hand the Liberal Democrats a super-majority in parliament.
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That would suggest the constitutional amendments campaigned for by Mr. Abe, who served as PM from 2012 to 2020 in addition to a brief stint in 2006, could become a reality.
Proposed changes to Japan's pacifist constitution would enable the nation to rebuild its army, neutered since the end of the Second World War, and play a much larger part on the world stage.
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Japan is the world's third-largest economy and a mainstay of the G7. Though its army, known as the Self-Defense Forces, is banned from major expansion by Article 9 of the United States-drafted treaty presented in 1947.
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News of the overwhelming victory came hours following the prime suspect linked to Mr Abe's killing was handed over to prosecutors earlier today.
Tetsuya Yamagami, 41, is an ex-member of the Japanese navy. He was arrested after being grappled to the ground instantly after the shooting in Nara, Western Japan on Friday morning.
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Abe was airlifted to the hospital but died of blood loss after being hit twice in the back. Video on Japanese television revealed a man quietly approaching Japan's longest-serving prime minister from behind and firing.
Wiry and bespectacled with shaggy hair, the suspect was seen stepping into the road behind Abe, who was standing on a riser at an intersection, before unloading two shots from a 40-cm-long (16-inch) weapon wrapped with black tape. He was tackled by police at the scene.
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After two days of questioning Yamagami was escorted out of the Nara Police Station to be taken to the local prosecutor's office on Sunday afternoon.
The horror attack on Japan's longest ever serving Prime Minister left the nation in mourning ahead of the survey.
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The Liberal Democratic Party expected a wave of backing in the wake of Abe's assassination.
Abe attempted to lift the economy out of chronic deflation with his bold "Abenomics" policies, beef up the military and counter China's increasing influence.
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As the country mourns, the Liberal Democratic Party and its junior coalition partner Komeito could earn from a potential wave of sympathy votes, political analysts announced.
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Yesterday, police seized DIY weapons discovered at the home of Yamagami's, including a five and a nine-barrel "shotgun" controlled by a mobile phone.
A top police official disclosed that security lapses did permit an assassin to get close behind and fire his gun at former Japanese Prime Minister Shinzo Abe while he was addressing a campaign rally. Yamagami jury-rigged the weapon from parts bought online, spending months plotting the attack, even attending other Abe campaign events, including one a day earlier some 200 km (miles) away, media announced.
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He had considered a bomb attack before opting for a gun, according to public broadcaster NHK. The suspect explained to police he made guns by wrapping steel pipes together with tape, some of them with three, five or six pipes, with parts he bought online, NHK stated.
Police discovered bullet holes in a sign attached to a campaign van near the site of the shooting and believe they were from Yamagami, police announced on Saturday. Videos revealed Abe turning toward the attacker after the first shot before crumpling to the ground after the second.
Unemployed Yamagami, 41, told investigators he acted because he believed rumors that Abe was connected to an organization that he resents, police announced. Neighbors described him as a loner who did not reply when spoken to.
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