The word "manifesto" is originally Italian, although it derives from the Latin "manifestum".

It means to make something public, clear or conspicuous, and that, I guess, is what Labour, the Conservatives, Liberal Democrats et al would claim to have being doing last week.

There was much comment on the cross-dressing therein. Labour made like the Tories, promising fiscal rectitude, while the Conservatives presented themselves as champions of the working class. The Lib Dems channeled the Wizard of Oz, pledging to give the Tories heart and Labour a brain.

This morning in Edinburgh it'll be the SNP's turn, and it seems their manifesto won't conform to type either. Instead of setting out Scottish-only policies, it will be "a programme for delivery on a UK-wide basis".

Given the tenor of the SNP general election campaign so far, this makes perfect sense, for no longer is "relevance" a problem for the party in a Westminster context. Indeed, the Nationalists are considered so relevant that the Conservatives can't stop talking about them.

And although endless talk of a "coalition of chaos" is supposed to be pejorative, intended to bounce English voters into backing the Conservatives, all publicity is good publicity as far as the SNP is concerned.

The SNP, as a press release observed over the weekend, is "contesting a UK-wide election entirely on its own terms". That is true, up to a point, although the party's campaign and strategy remains patchy and nowhere near as slick as its performance in the 2011 Holyrood elections.

Nicola Sturgeon is her party's chief asset. In both UK leaders' debates she demonstrated all her considerable strengths, and if anything was more impressive second time round. It's worth observing that her predecessor almost certainly wouldn't have come across as positively; where the First Minister exuded sweet reason, Alex Salmond would have blustered.

Indeed, Sturgeon's moderate demeanour has made the Prime Minister's doom and gloom seem like hyperbole, and although polls suggest the majority of English voters don't much like the prospect of the SNP influencing their next government, yesterday David Cameron tried a new tack of warning that "these people" (the SNP) wouldn't really "care" about people in the rest of the UK, who "wouldn't get a look in" when it came to major public spending decisions. This is unfair, but the game being played is obvious.

The TV debates were also important for the simple reason that they "cut through", or in other words leave an impression on voters who don't pay attention to the minutia of politics. As the commentator Janan Ganesh once put it: "The first law of politics is that almost nothing matters. Voters barely notice...the events, speeches, tactics, campaigns or even strategies that are ultimately aimed at them."

Elections, therefore, "are largely determined by a few fundamentals". Nicola Sturgeon's performance represents one such "fundamental" in this election, as does the SNP's typically pithy and well-targeted messaging. On Saturday the party unveiled a new poster that combined the two: a large image of the First Minister and her "vow" to "make Scotland stronger at Westminster".

As election messages go, it's easy to understand, while backing up the other key impression the SNP wants to convey, that it more than any other party truly "stands up for Scotland". And once voters are convinced that a certain party does that - as they were with the Scottish Labour Party until recently - then they'll generally forgive a multitude of sins.

Which brings me to the rest of the SNP's campaign, which is not as impressive. Somehow I find it hard to believe the party intended to spend so much time fire fighting over Full Fiscal Autonomy, but Alex Salmond left his colleagues and successor with little choice.

In early January the former First Minister told the Dundee Courier that this Westminster election would be about "delivering" Home Rule, that is "control of all domestic affairs and taxation", leaving just foreign affairs and defence reserved to Westminster. When asked if this was indeed an election aim, the SNP didn't quite agree, but then it didn't quite disagree either.

Nevertheless the die was cast, and no matter how hard they've tried, FFA won't go away. In response, as Jim Murphy observed a few weeks ago, Ms Sturgeon et al have given the impression of making things up as they go along. One minister, for example, told me a few days ago that FFA hadn't even "been defined"; only it has - in several Scottish Government papers.

The SNP's broader negotiating position should it hold the balance of power in a hung Parliament has also been badly thought through, if at all. Beyond making Labour and Ed Miliband uncomfortable, as the campaign's worn on the Nationalists have actually minimised their potential influence rather than maximising it.

As Ms Sturgeon reiterated on Andrew Marr yesterday morning, she "will not do a deal with David Cameron in any circumstances", which is politically understandable but leaves only one option: Labour. And even there, the party has ruled out formal coalition and steadily moved away from the idea of a semi-formal confidence-and-supply arrangement.

For that, given the SNP's insistence that non-renewal of Trident ought to be a "red line" issue, is more or less impossible. At some point Nationalist MPs would have to support a Labour Queen's Speech or Budget including provision for Trident, and that, according to the First Minister, simply isn't going to happen. So that leaves merely an issue-by-issue deal with a minority Labour government.

But even under that scenario the SNP has made it clear it would not do anything to facilitate a Conservative government, which one assumes includes failing to support Labour on a confidence issue. Now this, however one looks at it, is not a strong negotiating position. Sure, the SNP could hold Labour to ransom on certain issues, but it would hardly be conducive to stability or, more to the point, avoid the emergence of a Conservative-led alternative government. The SNP keeps wielding the Fixed-term Parliaments Act in response to this point, but that reduces rather than removes the prospect of a government falling. Ed Miliband might simply call the SNP's bluff.

Here, too, it looks as if the SNP has been making this up as it goes along. Neither this, meanwhile, nor FFA is likely to feature heavily in today's SNP manifesto; such documents are read by few outside the media-political fraternity, and only become important when a particular pledge is broken, say tuition fees in 2010 (but not, curiously, the SNP's promise to eradicate student debt in 2007).

There is a broader point: for all its talk of promoting "the good life", the Conservatives ultimately want a smaller state; for all its promises of fiscal rectitude Labour wants (one would hope) a more socially just society, while for all their non-constitutional policy agendas the SNP and UKIP want independence for Scotland and the UK respectively. Anything parties say and do during an election campaign is but a contrivance to further those central aims.