Labor has been the frontrunner in the published opinion polls for the past three years. This week was no different, with the tightening since the election was called reaching a plateau. But Bill Shorten has supplied a powerful campaign moment in an otherwise lacklustre affair. He spoke emotionally about his mother Ann’s life and its key motivating force for him. It was in response to an attack on him, and her career, in several Murdoch papers. And it is a development that has rebounded badly on his opponents.

The Coalition was mightily disconcerted by Labor’s campaign launch last Sunday and by Shorten’s follow-up appearance on the ABC’s Q&A the next night. It was widely seen as the Labor leader’s strongest performance of the campaign, where he convincingly answered audience questions for an hour. But it was his four-minute answer to the final question of the show that really worried the Liberals and their followers in the media.

To get the full context we need to go back to a feature interview Prime Minister Scott Morrison gave to The Sydney Morning Herald, where he spelt out how he would run down Shorten in the final fortnight of the campaign. He was confident he knew how to beat “shifty Bill Shorten”. All he would have to do was reinforce the negative perceptions voters apparently have of the Labor leader. Monday’s Newspoll suggested Shorten’s approvals were deteriorating in the campaign. The Ipsos poll in the Nine newspapers found the exact opposite.

But if Shorten is perceived to be “inauthentic” and “shifty”, a TV audience questioner gave him a chance to address it. He was asked: “What will your leadership culture be? How will your government guide all of us as a community in relation to our culture in being a decent and caring country to live in?”

Shorten took the question head on. He said: “My style of leadership is not that of I know best and everyone else must do as I say. I’m not a lone ranger. I’m not going to be a Messiah … I would rather say my leadership style is one of the coach.” He went on to speak about listening and taking advice, but it was his heartfelt references to his mother that struck a chord.

He told how she came from a working-class family and had to put on hold ambitions to be a lawyer to take up a teacher’s scholarship so she could go to university – becoming the first in her family to do so. She worked for 35 years at Monash University and enabled her son to go to university. Without spelling out the discrimination Ann had suffered as an older woman in the workforce, the opposition leader said: “I can’t make it right for my mum but I can make it right for everyone else.” To applause, he said: “When we’re equal, when we get equal opportunity, we are going to be the best country in the world, with no arrogance. That’s my leadership style.”

The partisan attack on the Labor leader opened the way for him to hit back at one of the Liberals’ biggest vulnerabilities: their failure to promote more women through their parliamentary ranks.

Midweek, Murdoch’s Sydney tabloid The Daily Telegraph went for broke. On page one, it ran a story under the headline “Mother of invention”, and set out to destroy what it said was hailed as Shorten’s “election-winning moment”. It accused him of omitting the fact his mother went on to enjoy an illustrious career as a barrister. The paper said he had failed to disclose that his mother graduated law later in life “and [practised] at the bar for six years”. It said the Labor chief had only told half the family story. If that were the case, however, he left out the half that gives even more potency to his mother’s legacy.

One senior Liberal wondered who was the genius on their side who thought it a good idea to prompt the Telegraph’s ill-considered and cockamamie attack. Gallery journalists confirm the “Libs were shopping the story around on Tuesday”. Melbourne’s Herald Sun, unlike its Brisbane stablemate, The Courier-Mail, refused to take it. Scott Morrison played the innocent bystander. He told reporters it was a “very upsetting story” and he can understand that Shorten would have been “very hurt by it”. That was an understatement. The opposition leader was furious.

For 10 minutes during a half-hour press conference on Wednesday, Shorten spoke of his mother’s achievements. Fighting back tears, he told of a woman in her 50s with grey hair, who, even though she topped her law school, could not get a law firm to take her on for articles. When she eventually got to the bar, she struggled for briefs – “she got about nine briefs in her time”. Far from fulfilling her dream, as the Murdoch hatchet job claimed, she went back to education. The partisan attack on the Labor leader opened the way for him to hit back at one of the Liberals’ biggest vulnerabilities: their failure to promote more women through their parliamentary ranks. Their most high-profile and credible woman, Julie Bishop, has quit. She won’t be at the party’s Mother’s Day launch on Sunday to support Morrison, the man who blocked her run for the leadership. Shorten says the experience of his mother – “the smartest woman I’ve ever known” – is why he believes in the equal treatment of women.

News Corp sources say the Tele has another story on their news file to throw at Shorten. It is highly defamatory and legally dubious. The desperation that led to the attack on Shorten and his mother’s memory may give them pause to think about running it. As one Labor campaign worker says, “It’s difficult to know where the government ends and News Corp begins.”

So far, swinging voters have been more impressed by Shorten than Morrison in their head-to-head debates. That element was lacking in the one prime-time debate of the campaign. Of the free-to-air networks, only the ABC broadcast it live. The commercial networks were not convinced it would generate enough interest for their viewers. Wednesday night’s format – of one journalist moderator and no audience questions – left the way open for both sides to claim either their man won or no one landed a “killer punch”.

ABC TV’s Leigh Sales went close to landing one in her interview with Morrison on Monday night. Her very last question encapsulated the structural weakness the prime minister brings to this campaign: papered-over ideological divides. She asked: “Who will have the upper hand in driving Liberal Party policy if you’re re-elected – the climate change sceptics who killed the [national energy guarantee], voted against same-sex marriage and orchestrated Malcolm Turnbull’s downfall, or the mainstream of the party?” His answer was terse and the pause after it telling: “I will.” May 18 will, of course, test that. If Morrison wins, the conservative-constrained policies – scant as they are – will be taken as vindicated. The “modern Liberals” will be vanquished and those Shorten describes as the “knuckle draggers” on climate and energy will have won.

That scenario isn’t convincing the various independents and minor parties vying for the balance of power in the senate. They are pitching their spiels in the framework of a likely Shorten win. The Greens are offering an alliance with Labor on environment. It has been firmly rejected. The old Nick Xenophon party, now the Centre Alliance, is hedging its bets on some of Labor’s crackdown on tax concessions. The other parties of the right – Clive Palmer’s and Pauline Hanson’s and Cory Bernardi’s – are more shrill in their attacks on Labor than the Liberals. Research by The Australia Institute late last month found that while both major parties have improved their positions in the senate, the crossbench will still be critical in passing or defeating government legislation.

The same institute has released new research that should give the crossbench and indeed the Liberals pause for thought. It found voters believe the senate should support Labor’s policy agenda. For example, 59 per cent say the senate should pass Labor’s increase of the tax rate for incomes above $180,000, compared with 25 per cent who say they shouldn’t. There was similar strong support for Labor’s policies to reduce emissions by 45 per cent by 2030 – 58 per cent agreed, to 22 per cent who didn’t.

This last finding shows the weakness of Morrison’s demands for Labor to fully cost its climate change policies, saying that otherwise you can’t vote for them. The electorate clearly believes the Coalition is not doing enough and Morrison’s attacks merely confirm his unwillingness to do more. Former prime minister Paul Keating hit upon that vulnerability when he said: “There’s the prime minister walking around with a lump of coal. Coal is a fossil. The prime minister is a fossil himself – a fossil with a baseball cap, but a fossil.”

The odds are Morrison soon will be consigned to the museum along with Australia’s other political dinosaurs.