* PM's ruling bloc takes solid majority - projections
* Ruling bloc with allies falls short of 2/3 of upper house
* PM Abe said to seek constitution revision during term
* Turnout seen below 50%, second lowest on record (Recasts with NHK projection)
By Linda Sieg and Kiyoshi Takenaka
TOKYO, July 21 (Reuters) - Prime Minister Shinzo Abe's ruling bloc won a solid majority in Japan's upper house election on Sunday, but his coalition and its other allies fell short of the two-thirds majority needed to push ahead with revising the pacifist constitution, a projection by public broadcaster NHK showed.
Abe, who took office in December 2012 pledging to restart the economy and bolster defence, is on track to become Japan's longest-serving premier if he stays on until November, a stunning comeback after he abruptly ended a first, troubled one-year term in 2007.
Turnout, however, looked likely to have fallen below 50% for the first time in an upper house poll since 1995, NHK said, a sign many voters feel they lack an attractive option.
Abe's Liberal Democratic Party (LDP) and its junior partner, the Komeito party, were assured at least 69 of the 124 seats being contested in parliament's 245-seat upper house, with 9 seats yet to be called, an NHK projection showed. That, together with uncontested seats, assures them a majority.
But NHK said the ruling bloc and its allies fell short of the 85 seats needed to retain the two-thirds "super majority" required to begin revising the constitution's pacifist Article 9 to further legitimise the military, a controversial step.
Abe, however, said the size of victory showed voters wanted to debate changing the charter for what would be the first time since its enactment after Japan's defeat in World War Two.
"Of course, we cannot take the timing as a given, but I would like to achieve it (constitutional reform) somehow during my term," Abe said on TV as the evening wore on. Abe's term as LDP president runs until September 2021.
Changing the charter 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.
Without the two-thirds majority, Abe is likely to try to lure other opposition lawmakers to back his proposal to enshrine the military in the constitution, but that could be tough with a lower house election certain between now and 2021.
Surveys show voters are divided over changing the charter, with opponents worried doing so would increase the risk of Japan getting entangled in U.S.-led conflicts.
Any change must be approved by two-thirds of both houses of parliament and a majority in a public referendum. The LDP-led bloc has a two-thirds majority in the lower house.
Abe pushed his LDP-led coalition as the best bet for political stability during his campaign.
Opposition parties focused on voters' finances, including a potential hit on spending from an October rise in the sales tax to 10% and strains in the public pension system due to Japan's quickly ageing population.
LDP Secretary-General Toshihiro Nikai told a TV broadcaster that he would support Abe if Abe wanted to seek a fourth term as ruling party president. That would require a change in party rules. "Of course, I'd support him. Because he has the support of the people," Nikai said.
Abe, however, said later that he was "not thinking at all" about running for a fourth term.
Abe had already led his party to victory in five national elections since 2012, helped in part by a fragmented opposition and low turnout by voters.
The biggest opposition force, Constitution Democratic Party of Japan, was set to increase its seats but remain dwarfed by the LDP, projections showed.
(Reporting by Linda Sieg and Kiyoshi Takenaka; Additional reporting by Kwiyeon Ha and Elaine Lies; Editing by Michael Perry, Christopher Cushing and Dale Hudson)
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