Senator Nick Xenophon has announced he will quit federal parliament to run in the South Australian state election.

The cross bencher, whose future in Canberra was already under a cloud given constitutional questions over his citizenship, says he would have made the announcement earlier if it wasn’t for a High Court case.

“This will be the toughest political fight of my life,” he told reporters in Adelaide on Friday.

“But I’m up for that challenge because I love our state, our people and I believe that if you are in politics you should be there to make a difference.”

Senator Xenophon said he would wait for the outcome of the citizenship case and he won’t be seeking re-election.

“It seems that Labor and the Liberals are more interested in fighting about who gets their snouts deeper in the trough, rather than fighting for real people.

“I’m sick of seeing this contest of low expectations.

“We have a Government that is tired and cynical, that has abandoned accountability and transparency – Oakden, for instance. It is a Government that has failed to deliver reliable and affordable essential services that we depend on for our core needs for our everyday lives.

“And I’m dismayed that the Liberal alternative doesn’t promise anything better.

“If anything, the Liberals seem to be driven by a sense of entitlement that they are overdue for their turn in office, for the perks of power, rather than a coherent vision to make South Australia a better place.

“Both sides disappoint me to the brink of despair.

“We have a Government that deserves to lose, and an Opposition that does not deserve to win,” he said.

Labor leader Bill Shorten congratulated Senator Xenophon on his decision, but not without sticking the boot in over his dealings with the Turnbull government.

“He’s a very decent and pleasant fellow to deal with, although I do understand that Nick Xenophon has done more dud deals with Malcolm Turnbull that anyone ought to put up with,” Mr Shorten told reporters in Mackay on Friday.

One Nation Senator Malcolm Roberts told Sky News he was a “clever and cunning politician” and it would be better for federal politics “to have him out of the way”.

Xenophon will run in the seat of Hartley

Senator Xenophon said he had spoken to his three federal NXT colleagues who have all supported his decision to run in the seat of Hartley.

“It’s where I shop, it’s where I live, it’s where I’ve been for decades,” he said.

“I am a true local.”

He said he expected both Labor and the Liberals to “pull out all stops” to prevent him from breaking into state politics.

“I expect plenty of dirty tricks.  Plenty of mud will be thrown, probably the kitchen sink as well.

“For my part, it will be a do or die effort,” he said. 

Defence Minister Christopher Pyne warned South Australians against voting for Senator Xenophon in the state election. 

“A vote for Xenophon risks 20 years of Labor,” Mr Pyne wrote on Twitter. 

The Labor state government has been in power for 16 years.

Vincent Tarzia, the Liberal member for Hartley where Senator Xenophon is challenging, put on a brave face following the announcement.

“Bring it on. I’ll keep working hard for the community!” Mr Tarzia wrote on Twitter.

Meanwhile, SA Labor said the announcement was a cynical attempt to take advantage of the “split in conservative politics”.

“Nick Xenophon’s announcement today shows that he wants to take advantage of the massive split in conservative politics in South Australia,” SA Labor said.

South Australia at a ‘cross-road’

Announcing his resignation, Senator Xenophon said South Australia was at a “cross road”.

He suggested the state had become a brain drain, with young people moving elsewhere to kick-start their lives and careers.

“We can continue sliding down a path of decline, where we’ve fallen behind the rest of the nation on so many key indicators, including high unemployment, a declining population – so many with talent, particularly young South Australians being forced to leave our state to seek opportunities elsewhere,” Senator Xenophon said.

Senator Xenophon served in state politics between 1997 and 2007 before moving to the federal upper house.

MPs’ reactions to Xenophon departure

Attorney-General George Brandis said he was “surprised and saddened” by the loss.

“Although we represent different political points of view, I and other members of the Government have always been able to engage constructively with Senator Xenophon across a range of important legislation,” Senator Brandis said in a statement.

“Personally, Nick and I have always got on very well and I have enjoyed his company and quirky sense of humour. I will miss him.

Deputy Opposition Leader Tanya Plibersek told a press conference Senator Xenophon was making “whatever decision he thinks is in his best interests”.

“I think at the end of the day, I think South Australians will remember this is the same Nick Xenophon who refused to stand up for properly funding South Australian public schools.

The New Daily has contacted SA Premier Jay Weatherill and Prime Minister Malcolm Turnbull for comment.

— with AAP