Today at PMQs we had a debate on the debate about the debates. Was it worthwhile? That’s debateable.

For the most part this was yet another PMQs that generated more heat than light. Miliband said Cameron’s excuses were “pathetic” and he is “chickening out” of a head-to-head clash during the campaign. Cameron called Miliband “despicable” for failing to rule out a deal with the SNP, accusing him of trying to “crawl into Downing Street on Alex Salmond’s coattails”. It was not a terribly edifying spectacle.

However there was one illuminating moment that provides a clue as to what is likely to be a recurring Labour theme in the last stretch of the campaign. About halfway through the exchanges, just as we were wondering what on earth the point of all this, Miliband said: “I’ll tell you why this matters: it’s because it goes to his character.”

Somewhat bravely for a leader whose personal ratings lag behind those of the Prime Minister, Miliband believes that he can beat the Prime Minister in a contest based on character. He wants the public to view the PM as an out of touch, arrogant, aloof and unprincipled leader, who has broken his promises in office and is more interested in protecting the interests of the few than the many. Other recent PMQs sessions fit this pattern – think of the recent sessions where he chastised the PM for breaking his promise on immigration or those dominated by the alleged tax avoidance practices of wealthy Tory donors.

In contrast, Miliband is increasingly seeking to present himself as a man of principle who will stand up to vested interests, regardless of the personal flak he has to endure. In an insightful and highly personal interview with The Guardian this weekend, Miliband said we should not “mistake [his] decency for weakness”. On last night’s BBC News, in an interview at his home, Miliband said he doesn’t really care what people throw at him as he is “much more resilient” than people think.

His wife Justine also made her first foray into the campaign with her first major televised interview. She believes the personal attacks on her husband are going to get worse, but she is up for the fight because it’s “about whether decency and principle count for something in political life”.

Miliband first explored these themes in depth in a speech last summer entitled ‘The Choice on Leadership’, best remembered for his admission that he is “not the candidate from central casting”. In that speech he portrayed himself as a candidate of greater substance and integrity than David Cameron, and that is a theme Miliband is increasingly revisiting. We can expect this to continue.

The early signs are that the debate about the debates is not damaging the Conservatives’ standing in the polls. If anything, the polls appear to be moving David Cameron’s way. It is therefore tempting to assume that this remains a ‘Westminster bubble’ story, irrelevant to the wider public. And on the narrow issue of the debates, that may well be true.

But Miliband is attempting to weave Cameron’s evasiveness on the debates into a wider narrative: one that says ‘you can trust me more than you can trust him’.

The reason why Miliband is so desperate for a head to head TV debate with Cameron is because he knows he will get no better platform to hammer home that message.