A week before the 2001 general election, Labour deployed a poster featuring Margaret Thatcher's unmistakable hairdo superimposed on the balding head of William Hague, then Tory leader. The caption read: "Get out and vote or they get in." It was a crude effort to inject some urgency into the party base in a campaign that everyone (rightly) presumed Labour had won already.

The late Philip Gould, New Labour co-architect, strategist and pollster, is reported to have commented that a more potent poster would have been Hague's face without the wig and a caption: "Seven more days before you can get this man off the telly." In other words, cruel though it may be, the Conservative leader could lose the election by being himself and Labour just needed to give him enough of an airing for that effect to take hold.

This precedent was cited to me recently by a Conservative strategist who believes Ed Miliband awaits a similar fate. The Labour leader's sorry personal ratings in opinion polls are the single greatest source of confidence in the Tory ranks, when a dispassionate view of the electoral map makes it hard to identify many seats David Cameron can add to his existing tally. There are several he can easily lose. On a purely technical level, it is hard to see the Conservatives doing better in 2015 than they did in 2010, and easy to see them doing worse. By extension, Miliband doesn't need much wind in Labour sails to drift into Downing Street.

In my column this week I looked at the way the Labour leader has gamed this situation – playing a difficult hand with a certain skill for which he doesn't often get credit. He could end up becoming prime minister by a combination of studied tactical caution and psephological accident.

Downing Street's task is to avert that accident. It isn't just the headline poll numbers that offer encouragement but the granular, qualitative research into quite how much people seem not to like Miliband. It is, say the Tories, a "blink test" thing. Folk have taken against him. When they hear him speak, they want him to stop. When they see him on telly, they want to switch over.

There is a standard focus group exercise in which voters are asked to come up with the word that spontaneously comes to mind when shown a picture of a party leader. For Cameron, a common response is "privilege". For Nick Clegg it is "confused". For Miliband it is "no". Incredulous laughter is not uncommon. The idea that this man will be prime minister is judged so implausible by enough people that somehow democracy will find a way to make sure it doesn't happen – that is the great Tory hope. They are counting on a "late break" – the electorate surging in their favour at the 11th hour when the reality of a Labour government comes into view. Inevitably, the CCHQ campaign will aim to provoke that surge with aggressive, negative targeting of Miliband's character and capabilities.

The Labour leader's team is braced for this monstering and thinking about ways to counter it. This deliberation is in itself a source of some reassurance in the parliamentary party. There was a long period of denial about the "Ed problem" at the top of the party. MPs were picking up on public doubts in their constituencies – and from friends and relatives – but Miliband's strategists were, reasonably enough, reluctant to acknowledge that it was an issue and squeamish about discussing it. After all, who wants to tell the boss that swing voters in marginal seats think he is a doofus. Especially when his self-image is of a visionary politician who is redefining progressive politics for the 21st century and imagining a new kind of capitalist economy with equality and social justice baked into its core functions.

Image management was dismissed early on in the Miliband project as a vulgar distraction from the core mission, the kind of shallow gimmicky stuff that Cameron engages in. The problem is that time not spent defining the opposition leader on his own terms is time afforded to his enemies to define the opposition leader on their terms.

More effort is now going into working out how to "frame" the leadership question. The starting proposition is that there is no point dressing Miliband up as something he isn't. He cannot do the stagey Blair charisma thing, nor the casual Cameron hauteur that passes for natural leadership in the modern Westminster lexicon. From that basis it follows that a campaign identity for the opposition leader has to be built on positive things people already think about him. Obviously the perception of weakness and peculiarity have to be neutralised but their opposites – strength, charm – have to be projected in plausible terms.

According to Miliband's allies, that means working with the fact that people generally sense that his heart is in the right place and that he comfortably outpolls Cameron on measures of "understanding the lives of people like me". (The Mail recently had a story that the plan was to make Ed "Mr Normal" – a line that a senior strategist describes as "utter bollocks".)

As I understand it, the aim is rather to pitch Miliband as a decent, modest man of principle as compared to the spivvy chancer Cameron. Or, as one friend of the Labour leader puts it: "Ed is the guy who wakes up in the morning thinking about how to make Britain a better, fairer country for everyone. No one, not even the Tories, thinks that is true of Cameron." It also looks very much as if a role is being carved out for Justine Thornton, Miliband's wife, who is by all accounts impressive in many ways.

Still, the scale of the challenge can hardly by overstated. It is late in the day to be writing the script that says who the leader is and what he represents. Most storytellers would have put that bit rather earlier in the narrative. By now, many people will have made up their minds. The Tories certainly think Miliband has been written off and that the harder Labour tries to sell him, the more allergic voters will recoil.

The comfort for Labour is that few people pay much attention to politics outside campaign season, so there is, in theory, still some time to build a positive vibe around Miliband. Also, Britain has a parliamentary system, despite media determination to make it presidential. The Tories may want the election to be Cameron versus Miliband, but there are a lot of people who head into the polling booth knowing that the one thing they don't want is a Conservative government.

To mobilise that cohort, Labour doesn't need to generate great outpourings of love for its leader. It probably does need to fend off the charge that he is utterly ridiculous or a menace to civilisation. That ought to be possible because, whatever flaws Miliband may have, those are not among them.