Tavish Scott: Chris Huhne will not sink Lib Dem chances

But that is history. A speeding ticket a decade ago has ended his career, caused untold misery in his family and may lead to a prison sentence. All because he did not tell the truth.

Huhne asked me to back him against Nick Clegg when Ming Campbell resigned as Lib Dem leader. His political strengths were considerable. He was more left-leaning than right. He had fought and defeated the Tories in a previously safe Conservative seat. His natural inclination was more supportive of what we had built in Scotland with a successful Lib-Lab coalition.

But friends worried about his enemies. There were plenty at an early stage. Leaders make enemies, so it is a good idea not to start with too many. Could he have led? Yes, but where? All that is now academic, as a by-election in his Eastleigh constituency beckons.

The two coalition parties are now at each other’s throats. Judging by social media and the reporting of the Westminster bubble, neither side can wait. By-elections involving a straight fight between the coalition parties is new territory for British politics. There were by-elections in Scotland during the eight years of coalition government between Labour and the Lib Dems, but never one that pitched the two governing parties head to head. Jim Wallace is a great peacemaker but had such an eventuality happened, the then deputy first minister might have had a challenge holding back first minister of the time, Jack McConnell.

It is the relationship between the backbenchers that will really suffer at Westminster. Ministers in coalition have to get on, because they have no choice but to do things together. But for the Westminster backbenchers, a by-election is red meat. Tories are smarting over boundary changes to constituencies that now will not happen. The U-turn by David Cameron over reforming the House of Lords was forced upon him by his backbenchers.

On the surface, a Lib Dem majority of 4,000 over the Conservatives at the last election – with Labour nowhere – should be an easy target for the Tories. But a number of factors make this a genuine contest. The Huhne debacle has been known for months. He resigned from the Cabinet in February 2012, before the English local elections. Even when voters in the Eastleigh constituency knew their MP was in a heap of trouble, they returned Lib Dems in every ward on the local council, which mirrored the same result in 2011.

The Lib Dems gained the last Labour seat on the council, beating an incumbent of thirty years’ standing. And they topped the poll in the borough for the police commissioner election.

Ukip are the party with the greatest media-inspired momentum ahead of the Eastleigh by-election. But their campaign takes votes from the Tories. This a fight the Lib Dems can win.

• Tavish Scott is Liberal Democrat MSP for Shetland