By Kevin Buckland and Rocky Swift TOKYO (Reuters) -Japanese government bond investors are bracing for a potential power shift in upper house elections this weekend that could end up worsening the country's already frail finances.

Markets are fretting that the outcome of the Upper House election on July 20 could strip the ruling coalition of its majority and spur an increase in fiscal spending.

The slump in Japan’s long-term bonds intensified Monday, pushing yields sharply higher in a move that puts global debt markets on alert.