CANBERRA, July 27 (Xinhua) -- The leaders of Australia's major political parties have hit the campaign trail ahead of the crucial "super Saturday" by-elections.

Hundreds of thousands of Australians will head to the polls in five by-elections on Saturday, four of which were triggered when the sitting Members of Parliament (MP) were forced to resign after it emerged their dual citizenship made them ineligible to sit in parliament.

Prime Minister Malcolm Turnbull and Australian Labor Party (ALP) leader Bill Shorten have spent much of July in the marginal electorates pushing their cases in the by-elections which have been described as a referendum on their leadership.

With the seats of Perth and Fremantle in Western Australia (WA) considered safe for the ALP and the fringe Centre Alliance expected to retain Mayo in South Australia (SA), the focus of the nation is on Longman in Queensland and, to a lesser extent, Braddon in Tasmania.

Polling in both seats, which are both held narrowly by the ALP, has Turnbull's Liberal National Party (LNP) ahead in Longman and only narrowly trailing the ALP in Braddon.

If the LNP is successful in either Longman or Braddon it would be the first time in 98 years that a sitting government has won a seat off the opposition in a by-election.

A failure by the LNP to win either seat would leave the government with a one-seat majority in parliament and on the brink of losing power.

Despite the strong poll results Turnbull on Friday said that history was stacked against his party, going on to declare that a general election would be held in early 2019 regardless of Saturday's results.

Shorten's position as leader of the ALP if they lose either seat has been the subject of intense speculation but Wayne Swan, the incoming president of the party, on Friday dismissed any talk of a change of leadership.

"What we have had, over a period of five years, has been an unprecedented degree of unity," Swan told News Corp Australia on Friday.

"We've got a very courageous policy program. We came within a whisker of winning the last federal election. The party absolutely understands the importance of unity."

Anthony Albanese, the man considered most likely to succeed Shorten as leader of the ALP, pledged on Thursday that he would not challenge Shorten for the leadership if Labor lost any seats on Saturday, a promise Swan said he believed.

However, Griffith University political commentator Paul Williams said the result in Longman would likely decide the future for both Shorten and Turnbull.

"Bill Shorten has the most to lose," Williams told the Australian Broadcasting Corporation (ABC) on Friday.

"So if Longman goes down for the Labor Party, Bill Shorten will go down too."

It is the second time in 2018 that Shorten's future has depended on the result of a by-election after the party looked certain to lose the Melbourne seat of Batman in March before Ged Kearney pulled off a stunning upset on election day.