Tim Horton of the Fabian Society kindly describes me as a "decent progressive", but to judge from the rest of his article he doesn't have much confidence in my colleagues (The Lib Dem myth, 18 March). I'm pleased to reassure him, and other progressives thinking of voting Lib Dem at the next election, that they will be in good company.

Horton's central argument is that under Nick Clegg there has been a "rightward shift" in our party, citing the "ditching" of a "raft of former spending commitments" and language about "tax freedom" that will appeal to those on the right. Yet under the same Nick Clegg the central message of our forthcoming manifesto will be fairness – whether in taxes, education funding, jobs or politics.

The Lib Dem policy of raising the tax allowance to £10,000 per year is described by Horton as "a juicy middle-class tax cut to sell in Conservative marginals around the country". Yet this tax cut, and the progressive tax increases which finance it, are hugely redistributive. A £10,000 tax allowance means that people on the minimum wage should no longer have tax taken out of their paypacket – surely something progressives should support? And the money for this tax cut comes overwhelmingly from high earners – ending the extra pension tax relief that higher rate taxpayers enjoy, taxing capital gains properly, and taxing those with properties worth over £2m.

On public spending, Horton says that Clegg "has insisted that his party would now be a more ruthless cutter of public spending than the Tories". It is true that we have been more specific about cuts than the Tories. But if abandoning like-for-like replacement of Trident, scrapping ID cards and culling wasteful government IT programmes is ruthless cutting, then I plead guilty.

Instead, the biggest Lib Dem spending commitment is on a social justice measure – an additional "pupil premium" to help the most disadvantaged children, funded by removing tax credits from high earners. This is an idea Nick Clegg floated before he was even in parliament.

Horton's ends with the claim that "the Lib Dems still have some way to go before they can claim the mantle of social justice from Labour". Yet in year one of an incoming Lib Dem government, a key priority identified by our treasury spokesman Vince Cable is a green jobs package, designed to tackle unemployment and invest in the long-term infrastructure. We recognise that the recovery is fragile and that the real social injustice would be to consign a new generation to the scrapheap of long-term unemployment.

Finally, Horton makes no mention at all of our fairness in politics agenda. We have argued for an effective cap on political donations, so that no political party in Britain can be bought by sectional interests: the two old parties have, not surprisingly, resisted.

Being a progressive is partly about tax and spend, and the Lib Dems have a good story to tell. But it is also about breaking open a decaying political system which has protected vested interests for too long. Only we can do that.