Last week, at their manifesto launch in Edinburgh, the Scottish Liberal Democrats' leader, Nicol Stephen, retired exhausted following an arduous 15 minutes of questions. The assembled hacks demanded extra time to discuss the party's opposition to a referendum on devolution and other matters.

Tavish Scott, the party's transport spokesman, was obliged to convene an impromptu news conference in order to soothe the brow of the beast. He did a pretty good job as it happened . . . of imitating the Rev Ian Paisley.

Aren't the LibDems prepared at least to negotiate on the referendum? No, said Tavish. Would they accept a referendum with three questions on it, one of which might be the Liberal Democrat option of federalism; ie, more powers for the parliament? No!

What about the Constitutional Convention mark II, which they propose to set up to give Holyrood more powers? Would they consider putting its findings before the people in a referendum? No! No! No! Truly, Tavish is now the Abominable No Man of Scottish politics.

The Liberal Democrats are in danger of making themselves look ridiculous. Correction, the Liberal Democrats are looking ridiculous over their opposition - under any circumstances, until hell freezes over - to a referendum on the constitution. This was always a very strange position for a party of constitutional reformers that has participated in numerous referendums over the years.

Whether on Europe, Scottish and Welsh devolution or the Good Friday Agreement, referendums are how we do things in this country. Over three decades, they have become the accepted means of resolving constitutional issues. What makes the Liberal Democrats think they can dictate otherwise?

Try turning the issue on its head. If, as the LibDem leader, Nicol Stephen, insisted last week the "real referendum is on May 3", what does that mean if the SNP wins? Does it give the Nationalists the right to break up Britain, begin negotiations with Westminster over secession, without first getting a further explicit endorsement from the people? Of course not.

People vote on all manner of issues in a General Election. You cannot simply infer a nation's attitude to one of the most profound constitutional issues in a generation from how many people voted Alex Salmond on the ballot paper. Many people in May will be voting to scrap council tax, remove Trident or because they don't like Jack McConnell's face.

The Constitutional Commission, chaired by the LibDems' former leader, Lord Steel, 18 months ago proposed a whole range of new powers for Holyrood, from fiscal autonomy to welfare and immigration. The Steel Report actually looks a lot more Nationalist than the SNP's own very tame manifesto published last week. Given the history of devolution referendums, it would be unthinkable to wrest so many new powers from Westminster without the people of Scotland being given a chance explicitly to endorse such a move.

Yet the referendum is the only issue upon which the ideology-lite Liberal Democrats refuse to compromise. In the eternally shifting sands of LibDem policy, where everything is provisional and principles are merely negotiating postures, their rejection of a constitutional vote is the one bedrock, the one issue upon which they will not be moved; it's the one item on which they cannot be bought. They aren't the Liberal Democrats anymore; in this election they are the No Referendum Party.

Ludicrously, they say they will not even sit down with the SNP, should the Nationalists be returned as the largest party, unless Alex Salmond abandons the referendum and presumably goes back to the old SNP policy of regarding an election victory as a mandate for independence. If he refuses, the Liberal Democrat ministers will forgo their ministerial cars, along with their red boxes, and sulk on the back benches, rather than allow Scotland to choose the constitutional arrangement which best suits it. It beggars belief.

Indeed, and most people simply don't believe the Liberal Democrats. Such is their form, on issues like road pricing, that no-one believes they will actually do as they say.

The Mail on Sunday ran a story yesterday claiming that a deal has already been struck behind the scenes with the SNP. This is denied fiercely by the LibDems, and seems to have more to do with SNP open-mindedness on the nature of the referendum than anything that's coming from the LibDems.

Alex Salmond has moved a long way in the past week. At the SNP's manifesto launch, he told the media that his "preference' was for a single-question referendum putting only the option of independence or the status quo. But he refused pointedly to rule out there being other options on the ballot paper. This was a pretty remarkable concession.

Look at the arithmetic. If the polls are to be believed, in any multi-option referendum, the Nationalists would lose. In the recent Times ICM poll, 88% of Scots said they wanted more powers for Holyrood, but only 27% wanted full independence. Of course, the precise level of support for Scottish independence is hard to determine because it depends very much on how the question is phrased. But most polling experts accept that, while a comfortable majority of Scots want more powers for Holyrood, only around one-third favour leaving the UK.

In other words, the Liberal Democrats are rejecting a constitutional referendum which they would almost certainly win and which would probably cement Scotland's place in the United Kingdom for the next decade. What are they waiting for? They should be claiming game, set and match.

But the LibDems say they will do nothing to endanger the Union. That the SNP would turn their joint administration into a battering ram for independence, by fomenting rows with London, to prepare the ground for 2010. But surely the Nationalists would do that anyway, whether or not there is a referendum. In fact, a referendum would surely encourage the SNP to behave as responsibly as possible to demonstrate to the people of Scotland that it is capable of running a responsible administration.

But where this becomes objectionable is in the way the Liberal Democrats seem to believe that they can have a veto over the parliament itself. This is a decision of Holyrood, not any particular party. We have had two referendums on devolution - 1979 and 1997 - and it is the height of arrogance for one party to decide, unilaterally, that there should not be a third before further radical change.

The precedents are too well established now for the LibDems to dump this. Their rejectionism is constitutionally unsustainable, morally insupportable and politically inept. If the Liberal Democrats really want to give Scotland a reason for rejecting them, this is the way to do it.

Read Iain Macwhirter also in the Sunday Herald.