One charge levelled at the Lib Dems – that we say one thing in one part of the country but another thing elsewhere – has always struck me as fairly laughable. In so much as it may have any truth, it hardly compares to the contrast between what Labour says in London and how it campaigned in Oldham East and Saddleworth.

But another charge – that we are, to an uncomfortably large degree, a vehicle for protest votes – has greater force. Within the support for the Liberal Democrats, there is, of course, a strong and principled core liberal vote, but it is not a large enough vote. To make electoral headway, the Lib Dems need to top it up with people who dislike Labour, the Conservatives, or established politics in general.

Voting for negative reasons of course also forms a large part of the support of the other two parties, which is why their campaigns mainly invite people to vote against the other side rather than back positive propositions. But the Lib Dems, more unusually, have also gained some votes from people who are broadly mistrustful of mainstream politics. Many of these voters have a strong civic conscience and an interest in policy. They want to vote, but they feel uneasy about the real-life decisions that governments must inevitably make.

By voting Lib Dem, they have previously achieved the ideal balance: discharging their democratic duty but never being burdened by any responsibility for the tough and often unpopular compromises and outcomes involved with the exercise of power. This section of voters, uniquely and somewhat perversely, voted Lib Dem precisely because they thought we couldn't win and wouldn't be sullied by government.

After 80 or so years out of power, that seemed like a safe bet. They are, understandably up to a point, bewildered and upset. The Lib Dems, in government, are making executive decisions, and whereas in perpetual opposition a party can make noises that seem to please most of the people much of the time, in office that is not always possible.

But there is another section of the electorate, and I met large numbers of these people during the general election. They shared many of the liberal ideals of the Liberal Democrats. They agreed with many of our policies. They liked Nick Clegg and Vince Cable. They wished us well. But they didn't vote for us.

Why? Because they did not believe we were a viable proposition; a serious party of government. They, understandably, believed that there were two governing parties in Britain, and the Lib Dems weren't either of them. At a crunch election, such as 2010, they felt obliged to make a choice between the two governing parties. The Lib Dems, for all our virtues, were considered to be a wasted vote.

One other consideration nagged away at them. The Lib Dems, they believed, were energetic community campaigners, well suited to local government and opposition politics at a national level. But could the Lib Dems make hard-headed, tough decisions in government? Could we have the resolve to get to grips with big issues such as the budget deficit and national security? The Lib Dems, after all, were widely perceived to be the wishy-washy party of well-meaning fence-sitters. And, out of office for almost a century, it was virtually impossible to provide compelling national evidence to the contrary.

Of course, I would like everyone who voted Liberal Democrat in 2010 to do so again. For the first time in my parents' lifetime, we are implementing Lib Dem policies and principles in government. We should be proud of that achievement, and of the role we are playing in taking our country forward. But I am a realist. The Lib Dems are no longer an obvious vehicle for delivering a protest vote.

So we have a peculiar and original task: the Lib Dems have to replace the people who voted for us because they thought we wouldn't be in power with the people who didn't vote for us because they thought we wouldn't be in power. This transformation will not be easy or fast. The former group will be much quicker to spot betrayal than the latter group will be to identify sustained competence in government.

The former group will search for disagreeable policy priorities or the inevitable compromises of office, and often find the evidence they require to look elsewhere.

The latter group will, in time, look for a less tangible political indicator: a sense of a mature party where the whole adds up to more than the sum of the individual policy parts. In other words, a credible party of government.

And the prize for the Lib Dems? Well, more support generally results in better electoral performance. But there's a prize that is almost bigger. That prize is a more ideologically coherent and resilient Liberal Democrat party. A home for all progressive and serious-minded liberals, who will see supporting the Liberal Democrats not as a wasted vote, but as the ideal vehicle for attaining their ambitions for our country.