Zac Goldsmith’s support for Brexit is a major political mistake

Politics is a cruel business. Boris Johnson's announcement that he will back leaving the EU was obviously taken purely for his own political gain. But despite this, it will probably do more to boost his career prospects than anything else he has done since becoming London mayor. Boris had a choice between power and principle and chose the former.

Meanwhile his aspiring Conservative successor for London mayor Zac Goldsmith took the opposite choice. Over the weekend Goldsmith also backed Leave but unlike his predecessor he did so purely out of a long-held political principle. In a fair world this would help him. Politics is not a fair world. Unlike Boris, who now looks closer to real power than ever, Goldsmith's decision will significantly harm his chances of ever getting it. In the choice between power and principle, Goldsmith chose principle and it will be a costly choice.

In a column in today's City AM newspaper, Goldsmith bravely sought to make an advantage out of his choice.

"I recognise that opinion in London is at best divided on this issue, and as a mayoral candidate, it would be easier for me to quietly U-turn," he wrote.

"But I didn’t get involved in politics to test every idea with pollsters, flip flop on the big issues and then carry on regardless once elected. That characterises my Labour opponent’s approach to politics, not mine."

While this is a decent attempt, the truth is that Goldsmith's decision as principled as it may be, will only harm his chances of becoming London mayor.

For a start, Londoners are overwhelmingly in favour of staying in the EU according to recent opinion polls. Over the next few months the EU referendum is going to be the biggest debate in British politics. As far as most Londoners are concerned, Zac has put himself firmly on the wrong side of the debate.

But Zac's decision also undermines his entire campaign message against his opponent. For months now the Tories have warned that a vote for Sadiq Khan is a vote for a dangerous experiment in which Londoners would be treated like "lab rats". Yet by backing Brexit, Goldsmith is now arguing for the biggest experiment with London's economic future in its history.

In recent campaign leaflets, Goldsmith has accused Khan of being a "radical and divisive" figure, but what is more radical and divisive than arguing for Britain's entire relationship with the EU to be torn up? What is more experimental than arguing that Britain could divorce from the EU without putting at risk a single job in London's financial centre?

As have previously reported, many senior London business leaders were already sceptical of Goldsmith's candidacy. His announcement this weekend will only have deepened that scepticism. While this does not directly damage his campaign in terms of votes, it does undermine his central message that a vote for Goldsmith is the "safe" choice in the mayoral election.

Goldsmith's decision is even more mystifying when you consider that he has spent the past few months standing arm-in-arm with the prime minister in an attempt to persuade Londoners that he would be a close working partner with the current government. Yet in the past few days he has publicly declared his intention to campaign against that same prime minister in the most important campaign of his entire premiership. Even before Goldsmith declared his intentions, insiders reported that Cameron was less than satisfied with his mayoral candidate. That dissatisfaction will only now grow.

There is a long-standing conspiracy theory popular among Tory MPs, that Cameron would secretly like Goldsmith to lose. That now looks more credible than ever. It also looks more likely to take place.