Japan's ruling coalition has retained its grip on the country's upper house but failed to secure a "supermajority" that would ease the path to constitutional reform.<br />'' <br />'' The outcome of yesterday's election means Shinzo Abe remains on track to become the country's longest-serving prime minister later this year, and shores up his mandate ahead of a tax hike in October.<br />'' <br />'' He fell short, however, of achieving a "supermajority" of two-thirds of the seats in the upper house, a result that could have helped him achieve a long-running goal of amending the country's pacifist constitution.<br />'' <br />''<br />''Abe's LDP and its coalition partner Komeito took 71 of the 124 seats up for grabs in yesterday's vote, accounting for about half of the upper chamber. The two parties already control 70 seats in the other half of the 245-seat chamber that was not being contested. <br />