Opinion surveys show Democratic Party leader Pier Luigi Bersani is front-runner among five candidates, followed by youthful Florence mayor Matteo Renzi, who has vowed to shake up Italy’s political establishment if he is chosen.
Voting booths will be open from 08:00 to 20:00 local time (07:00 GMT to 19:00 GMT) with results due around midnight (23:00 GMT). About 3 million party and non-party voters are expected to take part.
Bersani may fail to secure the 50 percent he needs for a first-round victory, which will mean a second round run-off will be held on December 2 to decide who will lead an alliance that is well ahead in opinion polls for the next election.
In a second round Bersani is likely to pick up the votes of third-placed left-winger Nichi Vendola, the openly gay governor of the southern Puglia region.
The party vote will eliminate a major element of uncertainty in Italian politics ahead of the national poll to choose a successor to Monti’s technocrat government.
While the slick and dynamic Renzi, 37, is much more popular across the general population, canny career politician Bersani, 61, is favored by party supporters who will decide the primary.
Both men reject the idea encouraged by international markets that the respected and sober Monti should return after the vote to continue his economic policies that have so far included unpopular spending cuts, tax rises and labor reform.
Protests on Saturday by tens of thousands of students and workers from across the political spectrum highlighted the levels of discontent among Italians grappling with recession and rising unemployment in the euro zone’s third biggest economy.
With Silvio Berlusconi’s center-right People of Freedom (PDL) party slumping in opinion polls to less than half its support in the last election in 2008, the center-left has a clear field to win the general election.
However there are still several elements of uncertainty presented by the dramatic rise of the anti-establishment 5-Star Movement, which is now second in polls for the next election, and the around 50 percent of Italians who say they are either undecided or will abstain.
The center-right is due to hold its own primaries on Dec 16, but PDL secretary Angelino Alfano said on Saturday that may no longer make sense after Berlusconi told reporters he was again thinking about running, deepening chaos in his party.
It is also still unclear what electoral system will be used for the national vote now expected on March 10-11, as politicians have been arguing for months over how to reform an unpopular electoral law that allows party leaders to hand-pick members of parliament.