“You can’t trust a word the Lib Dems say”: here is an entirely accurate summation tweeted to the world in 2013 by the new Liberal Democrat MP for Streatham, Chuka Umunna. Announcing his second defection in four months, Umunna last night posed alongside Liberal Democrat leader Vince Cable, whom he once astutely declared “can’t run away from his record as part of the Tory-led government or try to pretend Tory policies have nothing to do with the Lib Dems”. Just two years ago, he bitterly declared he “can’t forgive” what the Lib Dems have “done to my area”; indeed that he “could never countenance suggesting voters support Liberal Democratic or Conservative candidates on account of their Remain credentials – this would require turning a blind eye to the cuts to our local schools, the NHS and other public services instigated by both parties in government from 2010 to 2015”.

Everyone is entitled to shift their opinions: as John Maynard Keynes once put it, “When the facts change, I change my mind. What do you do, sir?” But what facts have changed about the Lib Dems since 2017? Both Cable and Jo Swinson, the frontrunner to replace him as leader, are unrepentant about the austerity they imposed on the country, and which Umunna all too recently passionately condemned them for. It gets better. At the end of April, a leaked document from Umunna’s Change UK revealed his then-party’s plans to wipe out the Lib Dems as an electoral force. Umunna has gone from seeking to politically extinguish a party to becoming its enthusiastic parliamentary representative in the course of seven weeks.

Farewell to Change UK – we hardly knew your name | James Felton Read more

When Umunna and his motley crew launched their burning skip of a political party, they relentlessly declared that “politics is broken”. Let me gently suggest that if this is indeed true, Umunna should cast his gaze at the nearest mirror (not something I imagine he is averse to doing anyway).

I first came across Umunna in 2008 when I was working in the parliamentary office of a then little-known leftwing backbencher by the name of John McDonnell. His head of operations, Simeon Andrews, a lifelong socialist with a penchant for goth metal who sadly died last year, stood as the left candidate to be Labour’s representative in Streatham. Along with Andrew Fisher, now Labour’s head of policy, I helped run his campaign, sadly doomed in an era during which the Labour left was virtually extinct. The right’s candidate was Steve Reed, now a shadow minister, while Umunna stood as a “soft left” candidate, dedicated to opposing Trident and New Labour’s policies.

But as soon as he was elected, he shifted rightwards, disassociating himself from the soft left Compass group he was once a figurehead for. Appointed shadow business secretary, he embraced comparisons with Blairite icon Peter Mandelson; by 2013, his political trajectory led one senior left-leaning Labour figure to somewhat harshly tell me privately that “Ed Miliband has helped create a monster”.

If politics is indeed broken, it’s because there are too many politicians who believe in little other than themselves: Boris Johnson is certainly a striking example of this tribe, but so too is Chuka Umunna. He claims that taking the risky leap out of one of the established parties absolves him of the charge of “careerism”. But in truth, it was driven by the arrogance of a man who once derided West End clubbers as “trash”; a man whose ego, easily flattered by sympathetic journalists and parliamentarians, convinced him that he really was Britain’s Macron, that his charisma and popularity would propel a new political formation to dazzling success.

Now his vanity has collided with political reality, and to survive he has leapt into the arms of a party he recently sought to destroy. But however comforting the Lib Dems’ recent surge may be, it is shallow: its brand of politics has no purchase outside of Brexit – walk around your town centre and you’ll find punters can’t name a single Lib Dem policy other than a second referendum. It will, in time, come crashing down to earth, and bring Umunna with it.

In the meantime, enough of this farce. Umunna was elected to parliament wearing a Labour rosette, on the back of a Labour manifesto, just two years ago. When the Change UK European parliamentary candidate David Macdonald defected to the Lib Dems a month ago, Umunna said it was “disappointing that this candidate has chosen to pledge allegiance to another party – he has let down his fellow candidates and activists”. The same goes for a man who, in part, owes his parliamentary salary to the Labour activists who sacrificed their precious time pounding streets, knocking on doors, and delivering leaflets to get him elected.

Umunna is now in his third party within four months and, rather than insulting the voters who opted for a Labour candidate, he should go back to the people and give them a final say. That he won’t is both cowardice and hypocrisy – and is exactly why Umunna is so emblematic of our broken politics.

• Owen Jones is a Guardian columnist