It wasn't so long ago that Shinzo Abe appeared on course to becoming Japan's longest-serving prime minister ever.

But after his conservative Liberal Democratic Party's crushing defeat in Sunday's Tokyo assembly election, Abe's iron grip on power has finally shown signs of slipping — a setback critics say is the biggest crisis he has faced since his sweeping return to power in December 2012.

"This is a very stern rebuke from the public that we must do some serious soul-searching about," NHK showed a grim-faced Abe telling reporters at his Tokyo office Monday morning.