It's a measure of Bill Shorten's inestimable confidence and Malcolm Turnbull's perceived political ill-health that Labor should dare promise a repeal of company tax cuts that have not yet been passed.

The Labor leader's brazen pledge also shows how polarised Parliament has become and how the inequality debate has so successfully entered public argument.

It is a rare moment indeed to have the two sides of government so enormously at odds on tax policy.

If the Government wrangles two more votes to legislate the slashing of corporate tax to 25 per cent, Mr Shorten would take Labor to the election arguing that Australia should be the first OECD nation to increase tax on companies.

It's a remarkable state of affairs that's elbowed out orthodox economic thinking — that lower corporate tax rates are generally good for the economy and good for jobs.

The Opposition Leader has calculated that the Prime Minister's authority is so sickly he would not be able to withstand a Labor "fairness" assault that pits big business against the Bettys from Blacktown.

If Malcolm Turnbull succeeds in his quest for a corporate tax cut, his celebrations may be short-lived. ( ABC News: Adam Kennedy )

What the Prime Minister had hoped would be his signature economic reform has already been defined by Mr Shorten as a "corporate tax giveaway".

And a Turnbull tax triumph, should it come, would be afforded no time for celebration. By Mr Shorten's crafty design, it merely becomes the siren sound for a longer, lethal game.

Pressure is on senators

Labor's repeal promise has intensified pressure on the two hold-out senators.

Derryn Hinch and Tim Storer, who is barely a week into the job, know that the outcome of their deliberations puts them on one side or the other of a fight that will be central to the next federal election.

Mr Turnbull's initial reaction to his opponent's repeal pledge is that Mr Shorten stands for "fewer jobs and less well-paid jobs", but this message already lacks the gritty cut-through required in an election campaign destined to be super-charged by the demonising of big business.

Combatting those groundswell resentments will be a Herculean task, evidenced by Mr Shorten's growing confidence.

Just a fortnight after announcing an audacious $11 billion strike on cashed-out dividends for retirees, he and shadow treasurer Chris Bowen announced a backdown with a "pensioner guarantee" that effectively exempts more than 300,000 people originally targeted.

Sorry, this video has expired Labor announces Pensioner Guarantee to tax reform

The political nature of the carve-out wasn't disguised.

"Malcolm Turnbull and Scott Morrison have nowhere to hide," Mr Bowen said.

"They've been running around with their scare campaign trying to scare Australian pensioners. Well, now they stand exposed as defending this tax concession, which goes overwhelmingly to Australia's wealthiest households with no alibi about pensioners, no alibi for their lack of willingness to take on this tax exemption."

It was a hard-headed repositioning on a policy that had supposedly been very carefully calibrated.

Another reminder to the Prime Minister that he shares at least one trait with his opponent: ruthlessness.