TOKYO (Reuters) - Japanese Prime Minister Shinzo Abe's ruling bloc won a solid majority in an upper house election on Sunday but his coalition and allies fell short of a two-thirds majority needed to begin revising the pacifist constitution, public broadcaster NHK said.

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 in office until November, a stunning comeback after he abruptly ended a first, troubled one-year term in 2007.