Momentum, believed the US president Lyndon Johnson, wasn’t “a mysterious mistress” but a “controllable fact of political life”.

And looking back on two electoral anniversaries that fall this week – 20 years today since the 1997 Labour landslide and a decade since the SNP won their first Holyrood election on Wednesday – that much is clear.

Almost as soon as Tony Blair became Leader of the Opposition in 1994 political momentum was on his side, while several months before the 2007 Holyrood election the same was true of Alex Salmond.

What of two forthcoming elections, local and general? Well, in spite of Theresa May’s studiously anodyne campaign momentum is clearly with her, even in Scotland and Wales. As well as damaging Labour in town halls across the country, the Prime Minister will be hoping for a 1997-style landslide come 8 June.

Momentum, of course, is relative. So even if the SNP (finally) takes Glasgow this Thursday and retains the lion’s share of Scottish seats next month, it’ll appear as if electoral currents are against them. This is, by and large, unavoidable, for if we take the 2015 result as “peak” SNP (and, importantly, peak Sturgeon), then the only way is down.

This comes as a surprise to some inside the Scottish political bubble. Within the space of two weeks some political observers have gone from predicting a 1997-style Tory wipe out in Scotland to grudging acknowledgement of a polling trend in their favour, despite the revival having been clear since last year’s Holyrood elections.

Scottish Conservatives detected a change in mood, a “clear Unionist feeling” as one senior official put it, shortly after the 2014 referendum, “a desire to stop the SNP”. But at the 2015 general election it was too soon for this to benefit them as the party still wasn’t seen as credible. After the election, however, they worked hard to identify and “model” No voters, the sort of detailed groundwork Nationalists have been deploying for years.

They then targeted two groups of voters, those who regarded themselves as Conservatives but didn’t think there was any point voting for them (“credibility” voters) and those, generally younger, who were attracted by Ruth Davidson’s leadership (“fashionable” voters). Both were “latent” groups of voters that required motivation. The Scottish Tories also started asking No voters who was best placed to “stand up” to Nicola Sturgeon, and increasingly they heard “Ruth Davidson” in reply.

Early last year this focus group and polling data began to take shape as a campaign for opposition. Davidson hammered home the message that Labour and the Liberal Democrats were “soft” on the Union while presenting herself as the only party leader able to halt the onward march of Nationalism. This paid dividends in May 2016 and has been the party’s strategy ever since.

Where Scottish Tories remain weak is on the next stage of their strategy, moving from being perceived as a “strong” party of opposition to a potential party of government. That, however, requires policies, and the party has been something of a policy-free zone for the past couple of years (although its recent local government manifesto, “Localism for Growth”, was a step in the right direction).

So, while the Scottish Conservatives are managing, to a degree, to control their political momentum, it’s becoming increasingly hard for Nicola Sturgeon to do the same when it comes to support for the SNP and independence. On both, recent polling seems to suggest a downward trend.

It’s important not to exaggerate this: the SNP will still “win” the General Election in Scotland in terms of both seats and votes, but the point is they’re likely to experience a knock, and that’s something they’re not used to. The first set-back was losing an overall majority in the Scottish Parliament last May, which although disguised by six Green MSPs proved a psychological knock.

The SNP’s loss of momentum is also clear from its campaign strategy thus far. When the snap election was first announced, the First Minister framed it as another mandate for a second independence referendum, but after two polls showed the Tories on 28 and 33 per cent she changed tack, saying the election was actually about “standing up to the Tories”. A statement on progressing the “will of parliament” was then quietly shelved.

In other words, Ms Sturgeon is now responding to events rather than controlling them. Consequently, her election narrative sounds shrill, a losing negative message rather than the (broadly) positive campaigns of old. The Conservatives, she keeps warning bleakly, mustn’t be given a “free hand to do whatever they want to Scotland”.

It’s a core-vote strategy, fine for retaining existing voters but hopeless for attracting new ones. Campaigning in Glasgow on Saturday (where any mention of independence was conspicuous by its absence), the First Minister was reduced to May-like mantras. Voters, she said, shouldn’t allow Scotland to be “dragged back by the Tories”, while a vote for the SNP would deliver “real and effective opposition” in the House of Commons.

Part of the problem is that nothing the SNP believes will increase support for independence actually has. Brexit, overall, didn’t, nor did Mrs May’s “now is not the time” veto (although it wasn’t actually a veto), or the prospect of “perpetual” Tory rule.

A messy and protracted Brexit process might change that, but it’s a double-edged sword. Look, says Ms Sturgeon, this constitutional upheaval has been economically ruinous, but more of the same as a result of independence won’t be. I suspect soft Yessers won’t really buy that, especially when all they’re being offered is Andrew Wilson’s “after 10 years of economic pain we’ll be back where we started”.

When it comes to Europe, the First Minister has now resorted to giving her personal view (rather than that of party or government) when asked about an independent Scotland’s future “relationship” with the EU. On the BBC a few days ago, SNP MP John Nicolson said “I hope so” when asked if full membership would be in his party’s manifesto.

Even more problematic is the lack of electoral opportunities following the General Election. Previously, SNP strategy focused on the period between the expected 2020 UK election and Holyrood elections in 2021, but now the electoral cycle has changed. And, once lost, political momentum is extremely difficult to regain.