'Mugwump' attack has Lynton Crosby's fingerprints on it

“Mutton-headed old mugwump”: the turn of phrase was typical Boris Johnson, but the strategy was classic Lynton Crosby – and a taster of the personal onslaught the Labour leader will have to endure over the next six weeks.

How prominent the foreign secretary was likely to be in the campaign had been questioned, with some senior Conservatives viewing him as gaffe-prone. Downing Street sources on Wednesday would only say that he would have “a clear role”.

But his character assassination of Jeremy Corbyn suggests Theresa May has decided to put him to work as an attack dog, albeit one with a detailed knowledge of the classics.

As well as attention-seeking “look-at-me name-calling”, as Labour’s John Healey called it, Johnson’s approach was carefully judged. He was deliberately playing on voters’ concerns about Corbyn – and more specifically his approach to Britain’s national security – which some Labour MPs say is an issue on the doorsteps.

Before the formal campaigns have even begun, the centrepiece of the Tories’ pitch to voters is clear: “strong and stable leadership”.

May is unlikely to look comfortable hunkering down with children in a nursery to read We’re Going on a Bear Hunt, as Corbyn did last week, or addressing rowdy crowds; but Tory strategists hope her sometimes robotic style can be a selling point if it’s just evidence of political steeliness.

And the flipside of that message, better delivered by the scrappy Johnson or the brutal Michael Fallon, is that Corbyn is idealistic, waffling and weak; better wielding a placard than in charge of the nuclear codes. The Tories believe that every day spent talking about the respective leaders’ personality traits is a good one. And nothing will be off limits.

During last year’s referendum campaign, in which Crosby was not involved, there were moments when the remain camp pulled their punches, unwilling to use the same brutal attack tactics on their colleagues as they had previously deployed against Labour.

Cameron slapped a last-minute veto on a campaign poster picturing Johnson, the public face of the leave campaign, in Nigel Farage’s pocket. The same visual metaphor was used with devastating effect during the 2015 general election campaign, to stoke fears that Ed Miliband would strike a coalition deal with Nicola Sturgeon.

This time, nothing will be ruled out. And by suggesting that Britain would be ready to join strikes against Syria if President Bashar al-Assad carried out another chemical weapons attack on his own people, the foreign secretary will also have been hoping to heighten the divisions within Labour.

Many Labour MPs, including the deputy leader, Tom Watson, backed Donald Trump’s recent military strikes on Syria as the right response to Assad’s aggression; Corbyn was opposed.

Labour strategists aren’t concerned about these attacks: they believe that Corbyn’s long record of making what they regard as the right judgments on foreign policy issues, including opposing the Iraq war as a Labour backbencher, will serve him well; and the perception of him as weak on national security is already “priced in”, so no new revelations will hurt him.

They also believe that the public are turned off by May’s gung-ho approach to Britain’s defence, with Fallon even suggesting that she would be willing to launch a pre-emptive nuclear strike. And they hope Corbyn will come across as reasonable, and thereby strike a chord with voters. Labour also intends to spend as much time as possible on the campaign trail talking about policy.

May faces challenges in a series of areas, including whether she will stick with the pensions triple-lock; how she will fund the creaking social care system; and why many schools are facing funding cuts.

Labour has been quietly testing a series of what senior strategists say are radical policies, through focus groups and polls, and are confident they will be very popular.

With broadcasters covered by strict rules during the campaign, the strategists hope to persuade the public to rise above the mud-slinging and look at Labour – and Corbyn – afresh.