World Bulletin / News Desk
Pahor, 53, a long-time Social Democrat and former Slovenian prime minister, has been head of state since 2012 and announced his intention to seek a second term in December 2016.
Nine candidates in total are running in what is the EU country's sixth presidential election. Mayor and former actor Marjan Sarec is thought to be Pahor's strongest challenger with opposition New Slovenia leader Ljudmila Novak in third. Milan Jazbec, a diplomat is also running as an independent.
Although mostly ceremonial, the election could be a barometer of public opinion ahead of a general election scheduled for 2018.
Sunday's contest can be won by a simple majority; in the case no candidate wins 50 percent, a run-off election between the top two candidates would be held in a few weeks.
Polls will open between 7 a.m. local time (0600 GMT) and 7 p.m. (1800 GMT).