The lengthy federal NDP leadership race officially entered the final lap Thursday afternoon when New Democrats were able to start casting ballots for their next leader.

The vast majority of the 128,351 eligible party members are expected to vote through the mail or online before the party's March 24 leadership convention in Toronto. Those who attend the convention — some 3,000 New Democrats are expected to turn up — will be able to vote there; others can opt to follow the proceedings online and cast their votes electronically, ballot by ballot, in real time.

But most will vote in advance, submitting online or mail-in preferential ballots, on which they've marked their first, second, third and subsequent choices.

With preferential ballots, as voters' first choices are knocked off the ballot, their second choices are counted and then their third choices and so on until one candidate emerges with more than 50 per cent of the vote.

The fact that most New Democrats will have voted long before March 24 will take much of the drama out of the convention.

Trailing candidates who are forced to drop out can still theatrically cross the floor at the convention to another contender. But most of their supporters will have long since decided independently where their votes will go and won't be influenced by the convention-day drama.

To have much influence over their supporters, trailing candidates would have to drop out and endorse a rival before the bulk of the advance voting has taken place.

That helps explain the timing of Robert Chisholm's endorsement Wednesday of Montreal MP Thomas Mulcair, the perceived frontrunner. Chisholm, a former Nova Scotia NDP leader, dropped out of the race before Christmas, citing his inability to speak French. Chisholm, whose campaign had barely begun by the time he dropped out, last week doubted an endorsement from him would hold much sway over his former supporters, regardless of the timing.

"I frankly question how much influence I would have in that regard anyway," he told The Canadian Press.

Quebec MP Romeo Saganash has also dropped out of the contest but has yet to endorse anyone else. However, his campaign manager and some other aboriginal leaders who had been supporting Saganash, have recently endorsed Ottawa MP Paul Dewar.

Others vying to succeed the late Jack Layton include former party president Brian Topp, Toronto MP Peggy Nash, British Columbia MP Nathan Cullen, Manitoba MP Niki Ashton and Nova Scotia pharmacist Martin Singh.

Mail-in ballots must be received by the party by March 19. Party members can cast advance online votes until the morning of March 23.