Kevin Rudd says he's doing all he can to stop Tony Abbott becoming the next prime minister but he won't comment on chatter about the Labor leadership.

Prime Minister Julia Gillard's position is under renewed pressure as MPs return to Canberra for the final fortnight of parliament before the September 14 election.

Labor has slipped in the latest Nielsen poll to trail the coalition by 14 percentage points, just three months out from the vote. And support for Ms Gillard as preferred prime minister is at its lowest level in a year.

Mr Rudd was mobbed by reporters when he arrived at Canberra airport on Monday but declined to comment on the poll results that suggest he could make Labor competitive again.

"I have nothing further to add on what I've said before," the former prime minister told the scrum.

"My purpose in being here in Canberra this week, and my purpose in travelling around the country in support of Labor members and candidates, is very basic.

"It is to do everything I physically can to stop Mr Abbott becoming the next prime minister of Australia because he's the single most extreme right-wing political leader that the Liberal Party have ever thrown up in their history."

Mr Rudd has said he will not challenge Ms Gillard for the leadership again. The PM says she will not step down.