Progressive parties win elections by promising to make things better, conservative parties by saying things used to be better and can be once again.
Fittingly for a party that's not quite left and not quite right, the SNP is trying to have it both ways.
The Nationalists' pitch to the voters ahead of May's general election is that they alone will stand up for Scotland and create a more prosperous economy and a fairer society.
And they will do this not by harking back to their own past but to Labour's. The once-great Labour Party, this narrative goes, has lost its way and perhaps even its soul. This began under Blair, was confirmed by Iraq, but the betrayal turned ostentatious in Labour's referendum alliance with the Conservatives. Here is a party so obsessed with courting the votes of Middle England and maintaining the Union, it will jettison any and all principles that get in the way.
For austerity and welfare reforms. Against free school meals and home rule for Scotland. This isn't your father's Labour Party.
This is a curious political mugging in that the SNP is stealing Labour's old clothes but not actually wearing them. The Nationalists lament Labour's rightwards drift away from sacred principles but show no eagerness to champion these values themselves. You will search in vain for SNP pledges to nationalise the commanding heights of the economy, repeal secondary picketing laws, or reintroduce prices and incomes policies. It is still unclear where the SNP stands on a 50% tax rate for those earning over £150,000. We may be some ways off common ownership of the means of production, distribution and exchange.
One matter on which the Nationalists do echo Old Labour is defence, where the party borders on pacifist, and in particular in its opposition to the nuclear deterrent. SNP MPs made Trident the subject of an opposition day debate in the House of Commons on Tuesday. The object was to embarrass Scottish Labour, many of whose MPs boycotted the debate in favour of a Westminster Hall session on the crisis-hit oil and gas industry.
The SNP calculates that the nuclear issue can be used as a wedge between new Scottish Labour leader Jim Murphy, a retentionist, and the Labour faithful, where there is more sympathy for scrapping the programme. It comes amidst UK Government moves to replace the submarine-based system with a new generation of nuclear warheads. The "main gate" decision has been kicked into the long grass only because the two coalition parties can't agree on what any replacement should look like.
But while some Labour voters are hostile to Trident, there are many who view the nuclear deterrent as a necessary evil. Labour has long been a multilateralist party and still bears the scars from its flirtation with unilateralism, a term synonymous for the current generation of Labour politicians with Michael Foot, defeat, and impotent rage at Thatcherism.
The more excitable amongst the Nationalist ranks charge - and, pray for Nicola Sturgeon, quite a few of them truly believe -- that Labour and Mr Murphy are enthusiasts of nuclear arms. Love 'em, they do. Can't beat a bit of nuclear annihilation. Faster, Pussycat! Kill! Kill! This confusion of moderate social democrats with the cast of Dr Strangelove is asinine but more importantly it fails to grasp the political tension at the heart of the Labour Party, which is often mistaken for left-versus-right but is actually instinct-versus-realism.
Labour people instinctively abhor weapons of mass destruction - and let's not pretend Trident is anything other than a delivery system for the wholesale incineration of human life - for the same reasons most people do but also for distinctively Labour reasons of economic priorities and scepticism about military power.
But Labour people are, on the whole, not pacifists and are realistic about the importance of deterrence in a hostile world and the international clout nuclear weapons lend. Just as important, they recognise that the voters are divided on the matter but, when pushed, aren't keen on putting their hands up and hoping the North Koreans follow suit.
A Panelbase poll for Wings over Scotland, a site that is doing more than most to understand why the last referendum was lost and how the next one can be won, illustrated the split in public opinion with 44% of Scots agreeing with a continuing nuclear deterrent against 36% opposing it. Even amongst SNP voters, almost four in ten back nuclear weapons in principle. This echoes earlier polling showing public support, if not approval, for Trident.
As Stuart Campbell notes glumly: "The uncharitable but logical conclusion from the findings is that Scots are barely any less militaristic than people in the rest of the UK, but they don't like having Trident in their own back yard."
I would quibble only with "militaristic". When it comes to Trident, Scots are hard-headed, not hard-hearted and it is this wary realism that guides Labour's policy.
Which is why Trident was an odd topic choice for yesterday's debate. With less than four months to go until a general election, the SNP is positioning itself against public opinion on an issue that ranks low on the list of most voters' priorities. This looks like a core-vote strategy to gin up The 45 -- especially the ex- and soon-to-be-ex-Labourites -- and convince the hard-left not to waste their votes on the Greens or the Socialists.
I say "looks like" because I can't believe the SNP would intentionally surrender the centre ground this close to polling day. It may be that Trident is so offensive to them, so totemic of the British state and its boundless arrogance, that they just can't help themselves. But it's bad politics and tells middle-of-the-road voters that the SNP does not share their priorities.
Jim Murphy and Ed Balls spent Tuesday, as did other Labour MPs, focussing on jobs in the energy sector amid plummeting oil prices. This was a smart move and will have painted a vivid contrast in the minds of voters. The Nationalists are far ahead in the opinion polls and given the ineptitude of Scottish Labour over the last few years they deserve to be. The SNP got to this position by governing from the centre and crafting their policies forensically to the needs and aspirations of Middle Scotland. They need to bear that in mind in the general election campaign. They may have an overwhelming poll lead but as Labour learned painfully in 2011 poll leads are easily squandered.
Mr Murphy had a good day yesterday. He can't be allowed many more.