The Eastern European country of 2 million people has seen debt climb since 2008 as various global crises battered economies, and Pahor's Social Democrats have lost some support as a result, the BBC reported.
His opponent is former Prime Minister Janez Jansa, who leads the center-right Slovenia Democratic Party. His popularity has risen despite promises he would implement government spending cuts and raise the retirement age from 57 for women and 58 for men, Euronews.com reported.
Pahor took over as prime minister from Jansa in 2008, but lost a confidence vote in September over pension reforms, triggering the election.
Polls published in the capital, Ljubljana, indicated Jansa could win back his role, but not enough to form a majority government, therefore requiring a coalition, the BBC said.