You don't have to come to Scotland to know that the Liberal Democrats face a drubbing when voters go to the polls here four weeks from now. Next month's elections to the Scottish parliament, the Welsh and Northern Irish assemblies and English local authorities are inevitably about many different things in many swirling contexts. In the broader British context, rather than on the night or even over the next four or five years, they are about whether the Lib Dems can survive as a major party and, if not, who benefits. And the first important answer to that question will be delivered in Scotland on 5 May.

In any conversation with any Scottish politician or politics watcher, one is soon reminded that these elections cannot be seen merely as a snapshot of local views on the Conservative-Liberal Democrat coalition at Westminster. They are about Holyrood not Westminster, they will insist. They are a verdict, for or against, on the four years of Scottish National party minority government under Alex Salmond since 2007. And many people will vote quite differently in these devolved elections to the way they would – or did – vote in a general election.

All this is true. It would be a travesty not to acknowledge that politics under devolution marches to its own drum, not Westminster's. A Scottish election is framed by very different, generally speaking more social democratic, assumptions than a UK-wide one – look at the pledges in the Scottish Labour manifesto. It is absolutely true that Salmond, who has an approval rating that any UK party leader would kill for, is the central figure in next month's Scottish elections. All the same, you can't understand these elections without taking account of the Westminster dimension either. They are framed by UK politics and they will in turn reshape UK politics. Nowhere is this truer than in relation to Nick Clegg's party.

Paradoxically, the Lib Dems aren't even very big in either Scotland or Wales in the first place. In 2007, last time there were devolved elections, the party came fourth in both contests. They got only one vote in six in Scotland and one in seven in Wales. Even so, it is the difference between what they got in 2007 and how they seem set to perform in 2011 that explains why the Lib Dems are so pivotal right now. In Scotland the Lib Dems have slumped from 16% in 2007 to 5% or 8%, take your pick, in the most recent polls. Meanwhile, in Wales they have gone from 15% in 2007 to 8% in Thursday's Western Mail survey.

These figures are not just dire. They do not merely raise the possibility that the Lib Dems will not end up with enough seats to be viable coalition partners, even supposing that Labour wants them, when the votes are counted. They also, crucially, pose the question of where these lost Lib Dem votes are now heading, and how that will affect the outcome. And they compel us to consider whether the Lib Dems can ever get them back, in England too, not just at the next general election but beyond.

Not surprisingly, something similar applies to their UK coalition partners. The Tories are being harmed in the Scottish and Welsh polls too. But neither the absolute nor the proportionate damage is on the scale of the damage being inflicted on the Lib Dems. In Scotland, the Tories are down by between four and six points from the 17% they scored in 2007. In Wales, though, they are down by just two from 22% four years ago. In short, Tory voters are significantly more likely to stick with their party than Lib Dem voters with theirs. And in any case there are more Tory voters in the first place. It could be worse for Cameron. But it could hardly be worse for Clegg.

The large fact, therefore, is this. In 2007, one Scottish voter in three voted for either the Tories or the Lib Dems. Eleven months after the formation of the Cameron-Clegg coalition, that is down to one voter in six. The Tories are defending their position reasonably well. But the Lib Dems are not. Unless they can turn things round, which no one to whom I have spoken believes they can, they are facing a disastrous 5 May and possibly even long-term oblivion.

These lost voters will certainly shape UK politics by their switches. In Scotland, it now seems clear, the gainer is increasingly the SNP. This wasn't so apparent in the immediate aftermath of the 2010 general election. Back then it seemed that Labour would be the electoral beneficiary in Scotland from the formation of the Cameron-Clegg coalition. By the turn of this year Labour looked set to oust the SNP from power by a large margin. But as polling day has neared, the SNP has steadily regained ground. Today the SNP and Labour are neck and neck. And Salmond has clearly outperformed Labour's Iain Gray in the early days of the campaign. Indeed so has the Tories' Annabel Goldie.

In contrast to Wales, where the only question for 5 May appears to be whether Labour can increase its share of the vote and seats to an absolute majority – yesterday's poll suggests it can – in Scotland the issue is a two-horse race. In both Wales and Scotland, Labour is running against the London coalition, posing the question: "Who can defend you best against the coalition's cuts?" In Wales the strategy is paying off. Yet in Scotland, plenty of voters – including plenty who will support Labour in a general election – are opting for the SNP. This is not about independence. That's still a non-starter, especially after the banking collapse. It is not even about whether the SNP will be able to balance the books. It is simply about seeing Salmond as Scotland's best champion.

It is too early to say who will win in Scotland. Labour may yet hold the SNP off, though whether Gray rather than Salmond will then become first minister is an intriguing question. As ever, electoral arithmetic and realpolitik will decide. Much will depend on the Tories.

But it is not too early so say who will lose. In both Scotland and Wales, liberalism has been a resilient and proud, and above all an anti-Tory, tradition since the days of Gladstone. Now those traditions are on a cliff-edge of destruction. Across the UK, the Lib Dem vote is proving much less resilient to the demands of the coalition than, confession time, I had expected. Can the Highlands, the Borders and Mid-Wales buck the trend and enable to old cause to hang on? Hard to believe. Yes, the Scottish election will be about Scotland and the Welsh election about Wales. For this generation of Liberal Democrats, however, the question is whether they will simply mark a point of no return.