Howard tips tough fight in Bennelong

The Liberal Party has a battle on its hands to retain the seat of Bennelong in an untimely by-election, former representative and prime minister John Howard says.

Mr Howard, 78, joined Liberal candidate John Alexander on a street walk through the northern Sydney seat ahead of the December 16 by-election, sparked by Mr Alexander's resignation amid speculation he was a dual citizen.

"We don't take this by-election lying down, it's quite hard," Mr Howard told reporters on Thursday.

A series of recent polls show Labor's Kristina Keneally is neck and neck with Mr Alexander.

Mr Howard, who held the seat of Bennelong from 1974 until he lost to Labor's Maxine McKew at the 2007 election, says he was proud of Mr Alexander's willingness to disclose his potential dual citizenship and resign from office.

The former prime minister took aim at other MPs who had been less forthright in admitting dual citizenship concerns.

"He's recognised that and he's put himself on the line, he hasn't been pushed to it, he hasn't been dragged kicking and screaming to it, and I admire him for that," Mr Howard said.

"Other people are hiding behind declarations, 'oh we've got a better authorisation process than the other side' - now that, in the end, will end in tears for people who make those claims."