“I genuinely think it is important people know Corbyn claimed in recent days he never met the IRA. You cannot trust this man!”

So goes the Tories’ favoured attack line on national security – delivered in this case, in uncharacteristically plain prose, by Boris Johnson on Twitter. The message – simple and direct – racked up more than 4,000 retweets: hardly Ellen DeGeneres at the Oscars, but not to be sniffed at.

One nil to Boris? Not quite. There is one glaring problem with his tweet – and, indeed, with the Conservative attack line on Corbyn and national security. That problem is Martin McGuinness, who appears alongside the Labour leader in the accompanying picture.

I genuinely think it is important people know Corbyn claimed in recent days he never met the IRA. You cannot trust this man! pic.twitter.com/vyP6fUo0iw — Boris Johnson (@BorisJohnson) May 27, 2017

Suspend for a moment the valid and necessary discussion over Corbyn’s links to republican paramilitaries, the lengthy catalogue of debatable statements on IRA violence, and his arguably peripheral role in the peace process as is commonly understood. As the Conservatives know better than any other party, what ultimately matters about lines of attack is whether or not they work.

That, ultimately, trumps their being morally watertight. Ed Miliband could tell you the same: there are no electoral prizes for true but unmoving PMQs jibes about your Tory opponent being the “prime minister for Benson and Hedge Funds”.

And so it is with Corbyn and the IRA. The terrorist sympathiser charge is most commonly expressed visually, through pictures of the-then backbencher alongside Sinn Fein’s Martin McGuinness and Gerry Adams. That much makes sense: the pair make for an easily recognisable shorthand. But that there is no consensus over what that shorthand is – especially in the case of McGuinness – means the Tories are failing to land what ought to be sucker punches.

Whether you buy the tenuous Corbyn line that he was in a sort of advance guard for the peace process, or believe instead that he was abetting the violent republican cause and excusing its appalling terrorism is irrelevant. So too the question of who or what McGuinness was, and how he ought to be remembered.

The question the Tories should be asking is how best to nail Corbyn on terror. The answer to that, uncomfortable though it may be for some, cannot be: “Look! It’s Martin McGuinness!” A YouGov poll taken on the day of McGuinness’s death revealed a plurality of people either believed the former deputy first minister of Northern Ireland (of which more shortly) should be either respected for his role in the peace process, or that condemnation for his terrorism and respect for his peacemaking ought to be given equal weight in any assessment of his character.

One in four voters did not know how McGuinness ought to be remembered, and the lack of consensus on the issue - and media focus on the broad sweep of his personal story rather than discrete acts of violence - means it may well be difficult to convince them. Another YouGov survey, taken last month, revealed that only one in five voters believed they knew a "fair amount" about Corbyn's republican links.

Are either of those numbers fair or right? Maybe not. But they speak to something bigger: that public opinion is too divided, too nuanced, too weakly informed for these attacks to take off. For better or worse, the McGuinness most people have seen the most of is the man who chuckled with Ian Paisley, governed Northern Ireland alongside his mortal enemies, shook hands with the Queen and was eulogised by Bill Clinton. The lay public’s memories of the Troubles have no softened to such an extent that he is now portrayed as a likeable, wisecracking wag in a Hollywood film about his relationship with Paisley. The same could well be said, to a lesser extent, of Adams's idiosyncratic tweets.

And there’s also the matter of McGuinness’s admittedly belated embrace of peaceful democracy. As many online have wasted no time in pointing out, there are countless pictures of Theresa May, David Cameron, Boris Johnson and indeed much of the cabinet alongside McGuinness. That the Good Friday Agreement and Provisional IRA’s disarmament is the key operative difference between the two barely needs saying. The difficulty, however, is that the pictures show a man who is different yet the same. It renders the visual messaging close to nonsensical.

One Labour MP told me some months ago that Corbyn’s association with the IRA would lose Labour seats directly affected by violence by default come an election. That may be so. But for better or worse, time has passed. There are other Labour voters who could and probably should should be repelled by Corbyn’s record on terror.

Folk memory, however, is a curious thing. Increasingly, referring to these people requires a degree of explanation - and if you're explaining, you're losing. McGuinness complicates a picture that ought to be - and frankly, is - clear-cut. Difficult though it is to argue that Corbyn was ever on the right side of history in Northern Ireland, by evoking the contested memory of paramilitary-turned-peacemaker Martin McGuinness, the Tories run the risk of making him look like he was.