The only hope for Labor is that one contender thrashes the other so convincingly that there is no chance of a comeback, and the loser slinks off to the backbenches as a loyal and silent trooper.
If Gillard succeeds, as most number counts predict, and does hammer Rudd back into a lead-lined box, she could finally focus on her agenda: work completed - an impressive list of difficult legislation including carbon and mining taxes, health reforms, education programmes, and means-testing of the Medicare tax rebate - and a long list of policies to come.
A flip-side victory for Rudd, assuming he inherits crossbench support, could allow Labor to present itself as laundered and ready to go anew, with a more popular leader who could expect a rapid repair job on unity to get on with the prime task of keeping Abbott on the Opposition benches.
The truth is that neither is likely to so completely refresh Labor. The party is tainted by bitterness and division, backstabbing and sabotage, with its successes heavily cloaked in public disorder.
Polls are unanimous in trashing the Government. While they show that Rudd is more popular than Gillard, Abbott's lead is so entrenched that Labor is unlikely to survive the election no matter who is in charge. Newspoll results give the Opposition a majority of 30 seats or more.
The intensity of the present battle is being couched in military terms even by its participants. Rudd, on his return to Australia yesterday morning after quitting as Foreign Minister in Washington during a heavy round of international visits, harked back to the Gulf War to describe Gillard's onslaught as "shock and awe".
But confirming his intention to challenge, Rudd did give one important assurance - if he loses the ballot, he will continue as an MP in the backbenches and will not challenge a second time. Gillard has also given that promise.
How those pledges would be honoured if sufficient pressure were applied will have to be seen, but if the loser does keep his or her word and the undermining stops, Labor would have a far greater chance of competing with Abbott.
Although no one has come forward yet and the prospect appears unlikely, a third candidate could emerge on Monday to complicate matters. Or, possibly, if desperate enough the party could turn to a more acceptable face - Defence Minister Stephen Smith has been mentioned - further down the track.
In the meantime, Rudd has reeled under a barrage of accusations and revelations by Gillard and her senior ministers that he said likened him to the "antichrist ... son of Satan"; Gillard in turn has been scorned for her polling disasters, and reviled as a backstabbing liar.
Facing the likelihood of inferior numbers in Monday's caucus showdown, Rudd has been using his considerable campaigning skills to present himself as an ill-used man of the people, turning with wife Therese Rein and daughters to social media and exhorting voters to swamp their MPs and local media with phone calls, emails and the like demanding they back him.
Gillard, in response: "This is not an episode of Celebrity Big Brother, this is about who should be Prime Minister."
But the picture Labor is presenting to the nation is of a toxic, incompetent management, tarred equally on both sides.
Gillard and supporters are painting Rudd's administration as dysfunctional and chaotic, frozen by months of paralysis, and led by a populist enslaved by opinion polls and headlines rather than conviction.
Senior Gillard backers said Rudd held his Cabinet in contempt, with tales of ministers vainly chasing meetings across the country from one stop to another until their issues appeared on the agenda.
Rudd is also accused of policy failure and of sabotaging Gillard, which he denies.
In reply, Rudd pointed to appalling polls and confirmed voters' views of Gillard as a two-faced assassin with his account of the 2010 coup. Gillard has always maintained she did not make up her mind until the day, despite recent revelations to the contrary. Fuelling the anger that turned so many voters off Gillard, Rudd said yesterday he had been given no prior notice of a challenge when his then-deputy fronted him and agreed to give him time to deal with a number of issues: she returned 10 minutes later and said "all bets are off".
Taking clear aim at voters, he said he wanted to return "to finish the job the Australian people elected me to do".
Rudd also accused Gillard of threatening MPs with their seats if they did not vote for her and criticised her actions on a number of policies.
Abbott has said he will not try to form a minority Government with the help of crossbenchers if Rudd beats Gillard but cannot secure the independents' support. Instead, he would seek an election - one which polls predict he would win by a landslide.
So now what happens?
* Kevin Rudd will stand against Prime Minister Julia Gillard in a Labor Party leadership ballot at midday (NZT) on Monday.
* If Gillard wins, a continued shaky slide to almost certain defeat at next year's election. If Rudd wins, a scramble to shore up a Government possibly without the guaranteed backing of key independents Tony Windsor and Rob Oakeshott.
* There is a chance he could replace them with other independents Andrew Wilkie - alienated by Gillard - and good friend Bob Katter, from north Queensland.
* Unless he can do that, he cannot visit Governor-General Quentin Bryce to form a new government - she will be in New Zealand for the week anyway - allowing time for new mischief from Liberal leader Tony Abbott.
