After the federal Liberal Party names its next leader Sunday evening, attention will quickly turn to how it can convince voters it doesn't belong as the third-place party in the House of Commons.

The Liberals will announce the winner at an event in Ottawa after a months-long leadership race that has left only one question: By how many votes will Papineau MP Justin Trudeau beat the second-place finisher?

Once that is settled, the new leader jumps straight into the job when Parliament resumes from a two-week break on Monday. He will face an early test in question period when he confronts his verbal sparring partner Prime Minister Stephen Harper, but his toughest job will be rebuilding a party that was reduced to a mere 34 seats in 2011.

To achieve that, the new leader has a key task to complete between now and the next federal election in 2015: reorganizing the party's grassroots, left in disarray after the last vote.

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Nelson Wiseman, a political science professor at the University of Toronto, said the party must convert its 300,000 new members and supporters it gained during this latest leadership contest into an active party base.

While that number is high, "that's ephemeral, that's just people getting online," Wiseman said.

"What you need are constituency associations that are functioning with the president, with the vice president, the secretary, the membership person, with the finance person, those sorts of things."

Wiseman pointed out that Trudeau has drawn huge numbers of supporters to campaign events and, perhaps more importantly, he's been "like a money-catcher," having raised more than a million dollars for his campaign.

"What they have to do is convert some of those people to actually take active organizational roles in the party themselves," Wiseman said.

"Supporters" did not require a membership fee and are a new category for the Liberals. Although it appears those who signed up as supporters may have a tenuous connection to the party, the contact list it created is the start they need for the rebuilding process.

"One of the big assets…is that they now have addresses for almost 300,000 people," Wiseman said. "And that's a big thing. You want to build up big data."

Meanwhile, Trudeau will be tasked with wooing back voters in time for the 2015 election, and the polls suggests he has a running start. More than one poll has shown the Liberals would win with Trudeau at the helm, although that's when respondents have been asked "if an election were held today.

A Nanos survey released Friday had support for the Liberals already at 35.4 per cent, Conservatives at 31.3 per cent and NDP fallen to 23.6 per cent.

Much can happen in two-plus years, and one of the immediate challenges will be to fend off the anti-Trudeau attack ads that reports suggest will be rolling out sooner rather than later.

As the leadership race wound down, Trudeau vowed he would not "go negative," and said he would stay away from attack ads if he became leader. During his speech at the Liberal leadership showcase last weekend, he criticized both the prime minister and NDP Leader Thomas Mulcair for being what he called "masters of the politics of division."

But if the Liberals stumble in the polls as the 2015 vote approaches, the tune on attack ads may change because "you go negative when you think you're going to lose," Wiseman said.

"Nobody runs for office and says, 'Elect me and I'm going to run negative ads.' That's just not a popular line."

However, what may provide the biggest boost to the Liberals' election fortunes is not what its leader does, but whether voters either tire of, or grow to distrust, the Conservatives after what will have been nine years in power.

"It's an old saw in politics," Wiseman said. "It's not so much that governments get elected, it's that governments get defeated."