Alannah MacTiernan reckons she knew Julia Gillard had started changing voter perceptions in Canning when she was doorknocking and came across a chap working on his Harley-Davidson bike.

"I don't think what they did to Rudd was right," the man told her.

But his wife opened the door to the house and said: "If you were a footy player or running a business and things weren't going right, you get rid of the person. The same with Rudd. I think Julia's the right person."

Before Ms Gillard's ascension and her mining tax deal, there were deep concerns whether Ms MacTiernan would make big inroads against sitting member Don Randall in the southern electorate.

But today's Westpoll shows a major swing to the ALP. Instead of sitting on a margin of 5.4 per cent, if the poll's results were repeated on election day, Ms MacTiernan would win.

And she believes the switch to Ms Gillard and the resolution of the mining tax are major issues in the turnaround.

But Ms MacTiernan admitted the asylum seeker issue was still resonating.

Mr Randall, who has held the seat since 2001, said the campaign was always going to be hard fought right up until polling day, given that both candidates had a high profile.

He said the asylum seeker issue continued to be the loudest in Canning, but added another "sleeper" - Ms Gillard's atheism.

"There's a large religious community across the seat and what I've heard is that there is concern about an atheist prime minister," he said.

Mr Randall said Andrew Forrest re-entering the mining tax debate this week was also likely to become a factor.