In this scenario, Societe Generale still expects 2.1 percent gross domestic product (GDP) growth in fiscal 2015, but the following year it would fall to 1.75 percent.
But if the prime minister's coalition loses the simple majority of seats, "Abe would likely resign, and this would be the end of Abenomics policies,"Societe Generale said. It expects this would also lead the Bank of Japan to taper its quantitative easing programs early and reform legislation would be very difficult to pass.
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"There would be a higher risk that the Japanese economy would fall into a diminishing equilibrium. This would mean a return to a deflationary environment," Societe Generale said, adding it would cut its fiscal 2015 and 2016 GDP outlook to 1.5 percent and 1.0 percent respectively.
Others are also concerned about how the seat tally comes up might affect Abenomics' prospects, including the monetary easing that has helped to drive recent market advances.
"If the ruling coalition were to lose a large number of seats in the election, the Bank of Japan would probably see this as an indication of strong public criticism of Abenomics, including the BOJ's bold monetary easing measures," Nomura said in a note Tuesday. "This might therefore make further monetary easing less likely."
Around 85 percent of respondents in the November Nikkei Quick monthly bond survey expect interest rates to rise if Abe's coalition comes up short, 85 percent expect equities to fall and 63 percent expect the yen will strengthen, Deutsche Bank said in its note.
—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1