I won't challenge Bill Shorten, says Anthony Albanese

Labor's main leadership alternative, Anthony Albanese, has guaranteed voters and his Labor colleagues that he will not mount a challenge to Bill Shorten even if the opposition loses ground at this weekend's "super Saturday" byelections.

With speculation growing that a rare reversal in the opposition-held seats of Longman (Queensland) or Braddon (Tasmania) would destabilise Mr Shorten's leadership, the Labor figure regarded as the most likely replacement provided an unqualified guarantee he would not seek to capitalise on the leader's misfortune.

Opposition infrastructure spokesman Anthony Albanese, flanked by Susan Lamb, the Labor candidate for Longman, on Thursday.Credit: Robert Shakespeare

Campaigning in the ultra-marginal seat of Longman with Labor's Susan Lamb, the man the government taunts as "the people's choice" due to his popularity among Labor faithful, was asked if he could offer "a 100 per cent guarantee" that he would not challenge.

"Absolutely," came his immediate response.

"What I'm concerned about is one thing and one thing only, being a minister in a Labor government ... I want to come back here with Bill Shorten as the prime minister and myself as the infrastructure minister."

Reporters covering the tight byelection race pressed further, with one then demanding a definitive "no" to the prospect of a challenge, which the NSW MP duly provided.

"Well I don't know how many times I can say it. Here it is, I'll say it really slowly - No. There you go. There you go in a word, it's not hard," Mr Albanese said.

Public and private discussion of Mr Shorten's leadership has intensified as Saturday's byelections have approached with polls suggesting Labor will struggle to retain either seat.

National opinion polls also show that while Labor continues to lead the Coalition, the gap may be narrowing and Mr Shorten's own standing as preferred prime minister languishes markedly behind that of Malcolm Turnbull.

Heading into the weekend, Labor has history on its side because sitting governments invariably experience swings against them in byelections at an average of 5.5 per cent on primary votes and 3.8 per cent on two-party preferred.

But that history has only increased the risk for Mr Shorten because the individual races are so tight and the circumstances that forced the byelections - Labor's dual-citizenship problems - suggest defeat is a serious possibility.

Leaders and senior frontbenchers from both sides have swamped the two regional seats in the final days of campaigning and Mr Turnbull is hopeful of becoming the first prime minister in nearly a century to wrest a seat from the opposition in a byelection.

Sources on the ground in Braddon, the south-western Tasmanian seat held by Labor's Justine Keay by a 2.2 per cent margin before she was forced to resign and recontest, say the seat could go either way with the preference flows of independents likely to decide the result.

Insiders say the level of political engagement by electors is surprisingly low, despite intense campaigning and a cavalcade of national political luminaries visiting the area during the 9-week campaign.

In Longman, in Brisbane's outer north, the Australian Electoral Commission has revealed some 25,000 electors have already cast their ballot via pre-poll voting.

But experts say the result may not be known on election night due to the wide field of 10 candidates, complex preferences flows, and the unknown size of the vote for Pauline Hanson's One Nation.