Japan ruling party lawmaker Mieko Nakabayashi isn't just worried that her Democratic Party will lose power in next month's election; she fears a comeback by rival conservative Liberal Democrats will spell a return to the prolonged one-party rule that critics blame for many of the country's past policy ills.

Three years after the Democratic Party of Japan (DPJ) ended more than half a century of nearly non-stop Liberal Democratic Party (LDP) rule, surveys suggest disappointed voters will hand the LDP the most seats in a December 16 poll for parliament's lower house. That would put LDP leader Shinzo Abe in pole position to form the next government and regain a job he quit in 2007.

"The risk is that the old LDP will come back and once that happens, Japan will take a long time to change back," said Nakabayashi, a former academic elected in the 2009 vote that propelled the Democrats to power for the first time.

"The DPJ could be an opposition party forever," she said, in an interview in her office near parliament.

Critics say the LDP's long reign created a triad of ruling politicians, bureaucrats and vested interests such as farmers, doctors and big businesses that rigidified over time, keeping Japan from responding to global and domestic changes.

Not all agree Japan's experiment with alternating power between two big parties is over after just three years, though some pundits agree the Democrats' demise can't be ruled out.