Understandably, Alex Salmond and his administration have had much to say recently about the democratic mandate and the will of the people but in the case of a new mass transit system for the nation's capital, the SNP has been caught on the wrong side of the argument. It is true that the Nationalists achieved their wafer-thin majority on the back of a manifesto that included major road and rail investment in the north of Scotland. To pay for it, the party proposed to scrap Edinburgh's tram scheme and the capital's airport rail link, saving a notional £1.1bn. An announcement to this effect is expected shortly.

Understandably, Alex Salmond and his administration have had much to say recently about the democratic mandate and the will of the people but in the case of a new mass transit system for the nation's capital, the SNP has been caught on the wrong side of the argument. It is true that the Nationalists achieved their wafer-thin majority on the back of a manifesto that included major road and rail investment in the north of Scotland. To pay for it, the party proposed to scrap Edinburgh's tram scheme and the capital's airport rail link, saving a notional £1.1bn. An announcement to this effect is expected shortly.

Though there has always been a vocal minority against the tram plan, it is generally popular. Invited to treat the recent elections as a referendum on the subject, at both parliamentary and council levels, a large majority of Edinburgh voters supported parties backing the idea. To press home their point yesterday, politicians from the other four parties represented in Edinburgh, plus independent MSP Margo MacDonald, joined forces to launch their Don't Trash our Trams campaign. This is surely an idea worthy of serious consideration. There is both a sound business case and a significant environmental case to support it. A successful 21st-century city requires an efficient transport infrastructure. A tram network that could be completed by 2010 would provide a mass transit system capable of cutting congestion and reducing carbon emissions, especially during the Edinburgh Festival. Preparatory work has begun and around £100m has been spent or committed; a sum that would be wasted if the project were abandoned. It is inconceivable that, as Mr Salmond has hinted, costs would spiral as dramatically as in the Holyrood building project, where specifications changed significantly during construction.

Mr Salmond's contention that trams could be replaced by extra buses is simply unworthy of him. There has been much warm rhetoric about consensus government and listening to other parties. Indeed, given the parliamentary mathematics, that is the only way forward. Here is a test. If Mr Salmond cuts the Edinburgh tram project as part of his budget, he risks the "nuclear option" of having the entire package rejected.

In the case of the city's other big infrastructure investment, the Edinburgh Airport Rail Link (Earl), the argument is much less clear-cut. The "Rolls-Royce" option, with its problematic tunnel under the runway and the creation of a rail hub directly serving two-thirds of Scotland's population, is also opposed by the Tories and the Greens. Few believe the budget will not grow like Topsy. A spur to the airport, as suggested by Mr Salmond, would be much cheaper, so freeing resources to tackle Scotland's sadly neglected rail network north of the central belt. Rather than falsely linking Earl with the tram project, the SNP should consider a quid pro quo. Now he is in the driving seat, Mr Salmond has a duty to rule in the interests of the country as a whole.