Japan is facing the prospect of another change in prime minister for the sixth time in four years.
Current prime minister Naoto Kan is facing a challenge from within his own party by Ichiro Ozawa, a veteran MP known as the shadow shogun.
Mr Kan has been prime minister for only three months.
The ABC reports that if Mr Ozawa wins, he would become the third prime minister in a year.
Mr Ozawa is seen as a powerbroker who normally stays in the background.
Meanwhile, the yen is at 15-year highs.
Tokyo is also embroiled in an escalating diplomatic spat with China over a collision involving the Coastguard and a trawler near disputed islands just east of Taiwan.