This time round there can be only one winner: The stakes are high for both coalition party leaders in Eastleigh

Inside Westminster

It was not supposed to be like this. When David Cameron decided to press ahead with a Bill to legalise gay marriage, he was confident of sending a positive signal to the voters that the Conservative Party had changed.

On Tuesday, 137 Tories voted against the measure, only 126 supported it and another 40 abstained. Remarkably, Mr Cameron was in a minority in his own party. He had, unwittingly, advertised the very opposite of what he intended: his party has not changed anything like as much as he hoped.

“The lesson of the 2010 election was that we didn’t modernise enough,” one Cameroon said. “Many of our MPs act as if the voters were clamouring for more right-wing policies. They think they are the solution. The truth is that they are the problem.”

The Liberal Democrats sniff an opportunity. Entering a coalition with the Lib Dems might have helped the Tories detox themselves. But if Mr Cameron cannot take his party with him, it is easier for Nick Clegg to position his party as the Coalition’s conscience. As one Lib Dem minister put it: “There are two Conservative parties – Cameron’s compassionate version and the nasty party.”

Although the Lib Dems face a difficult by-election in Eastleigh on February 28 after the Chris Huhne’s resignation, Mr Clegg’s party has a spring in its step.

One reason is the arrival of Ryan Coetzee as Mr Clegg’s director of strategy last autumn. He is a street-fighting number-cruncher who has brought discipline to an often incoherent Lib Dem message. He is a tough cookie more suited to an election run-up than his more cerebal predecessor Richard Reeves, former head of the Demos think tank, who has moved to the United States.

Mr Coetzee is from South Africa, where he was both an MP and election strategist for the Democratic Alliance, the Lib Dems’ sister party. He was warned that independent-minded Lib Dem activists would not like being told to be “on message.” In fact, they welcomed clearer marching orders from the top.

The one-time protest party is growing up as it tastes power, as Mr Clegg always hoped. Indeed, the Lib Dems often seem more grown up and disciplined than the Tories, whose MPs have lapsed into mad plots against a leader even though he is more popular than his party. The Coetzee strategy was evident when Mr Clegg leapt out of the starting blocks with an attack on the Tories over tax to launch his Eastleigh campaign. Mr Coetzee’s extensive polling shows that the public regards the tax threshold rise as a Lib Dem rather than Conservative policy. We are going to hear an awful  lot about this £9bn Lib Dem tax cut before the next election. “We have to show where we have made a difference inside the Coalition,” said one Lib Dem source.

The first head-to-head in a Lib-Con marginal since 2010 will strain relations between the Coalition partners. There can only be one winner in Eastleigh; the parties cannot match-fix a draw as they regularly do in policy disputes. The next few weeks may get nasty but Tory MPs who hope the Coalition will collapse will be disappointed. It has firmer foundations than they realise.

The Eastleigh stakes are high for both party leaders. A Tory victory would give Mr Cameron the breathing space in his own party that his promise of a Europe referendum failed to achieve. It would be hard for the plotters to argue he is not a winner. Conversely, a Tory defeat would remind their MPs that Mr Cameron failed to win a majority in 2010 and heighten their fears that he might fail again in 2015.

It would be easier for Mr Clegg than Mr Cameron to shrug off a defeat in the by-election. Although the Lib Dems have a strong base locally, they could attribute losing to a “Huhne effect.” However, a Lib Dem win would give the party hope that being in power might not mean an electoral bloodbath in 2015 after all.

The Lib Dems are targeting the 20 per cent of the electorate who are undecided, six out of 10 of whom would consider backing the Lib Dems. Many will be “soft Conservative” supporters in the south of England. This will be the key battleground between the Coalition parties in 2015, so Eastleigh will give us a fascinating preview.