It was an election in which many felt uninspired by the two candidates on offer. They represented a center-left party with flagging popularity, and a right-wing alternative that many feared would lead to social division and injustice.

This isn't the United States in 2016, but the UK six years ago, on the eve of its 2010 general election. And in Britain, those conditions led to an unexpected outcome: the sudden rise of an alternative third party.

Reporting on the Republican and Democratic conventions in Cleveland and Philadelphia this month, I was struck by the similarities. In 2010, the British electorate was angry about declining standards of living, and fearful of the consequences of mass immigration.

There was no British Trump at that time, but the Conservative Party was still reviled by many as the party of inequality and little compassion. One Conservative politician had warned that the public saw them as "the nasty party." (That politician was Theresa May, who just became Britain's second-ever female prime minister.) It felt as though the only thing that everyone could agree on was how much they feared the other party.

And then, to everyone's surprise, the laws of politics broke. Nick Clegg, the leader of a small third party, put in an unexpectedly strong performance in a TV debate, proposing radically liberal — even socialist — solutions to the country's problems. And a third party crashed through into the mainstream.

Overnight, Clegg's Liberal Democrats more than doubled their approval ratings. Membership boomed. Clegg himself became a social media star. And in the election a few weeks later, the Liberal Democrats did the unimaginable, surging ahead to win the third-most votes, behind the Conservatives and Labour.

And that is where things began to go wrong, from the point of view of Clegg's new, progressive supporters. Neither of the leading parties won enough seats to comprise a majority, and Clegg opted to join a coalition government, helping David Cameron's right-wing Conservative Party into office. 

The Liberals' promises quickly fell away. Breaking a solemn campaign pledge, they voted in higher fees for university education. Criminal justice reform scarcely happened. Funding for elder care was cut. 

And Clegg's supporters never forgave that betrayal. In the next election, the Liberals lost all the gains they had made, and became an even smaller force in parliament. Clegg was forced to make a humiliating video apology for all his broken pledges. It was remixed by angry former supporters into a song that went viral.

Many still haven't forgiven him. 

As Bernie Sanders and Elizabeth Warren may one day discover, it's not their political opposition who activists stay angriest at — it's the people they feel let them down.  

From PRI's The World ©2016 PRI