Steve Richards: An original thinker who realised that the public has lost faith in politicians

James Purnell joins the departing herd. The so-called Blairite Labour MPs were hopeless at acting together when fighting political battles, but they move as one in leaving the Commons at the next election. Purnell joins other former cabinet ministers, including Alan Milburn, Steve Byers, and Patricia Hewitt, who have opted to pursue alternative careers, a route taken by Tony Blair on the day he left No 10.

As a relatively youthful figure Purnell's departure is the one with wider implications. He had a future and chose to turn away. A few senior Labour figures saw him even as a possible successor to Gordon Brown, although Purnell was always adamant that he had no interest in the leadership.

His departure is a blow to David Miliband, a contemporary who stays behind to fight for the leadership after the election. Miliband worked closely with Purnell when they were advisers to Blair in opposition and in No 10. At a Labour conference fringe meeting a couple of years ago, when there was feverish speculation about a coup against Gordon Brown, I asked Purnell how often he spoke to Miliband. "We speak most days ... sometimes several times," he said with a provocative evasiveness. The coups went nowhere but Miliband has lost an ally in the battles to come.

So close to an election, Purnell's announcement is bad news for Gordon Brown. Another high-profile departure conveys a sense of impending doom whether it is intended to do so or not. Youngish MPs do not leave Parliament when sunny days of stimulating power appear to stretch ahead.

Labour's opponents will also argue that the departure of another former minister associated with Blair shows the party is moving leftwards. The reality is more complicated. Purnell never exuded a great appetite for party politics. I am not surprised that he is leaving the Commons and predicted he would do so when he resigned from the Cabinet in the summer. He is close friends with another former adviser to Blair, Tim Allen, who runs a thriving consultancy company. Perhaps Purnell will seek a more financially rewarding role by following a similar path. He has also shown considerable support for non-party bodies such as London Citizens. Next week Purnell is taking part in a five-day residential training course in Hemel Hempstead with 50 civil society leaders, including teachers and nurses, under the auspices of Citizens UK. The Conservative leadership is showing considerable interest in the organisation as well.

Purnell's limited hunger for party politics was matched by an indifference to the manoeuvring and scheming that are an unavoidable part of mainstream politics. His cabinet resignation was announced without consulting other disaffected colleagues. If he had wanted to bring about the downfall of Brown he might have let others know. He did not do so. Brown stayed and the former rising cabinet star left for a somewhat shapeless career as a backbencher and a thinker. Soon after he left the Cabinet, Purnell became more actively involved with the think-tank Demos, another organisation which now has close ties with the Conservative leadership.

Purnell was proving to be a more interesting thinker than the stereotype might have suggested. He was moving on from the Blair era. In a recent article he argued that New Labour had been paralysed by caution and went too far in its accommodation with markets. Purnell also formed a relationship with the leftish MP, Jon Cruddas. The two of them sought common ground partly to avoid a civil war in the event of Labour's defeat and, more constructively, in the hope of establishing a more pluralist culture in their party and beyond. Cruddas is another potential leadership candidate who will be disappointed by Purnell's decision to leave the front line.

His departure is both bigger than it seems and less significant. On one level it is the personal decision of another Labour politician with only a limited commitment to the vocation. Blairites never gave the impression they were ready for the long haul. Most of them did not survive in the Cabinet very long even when Blair was Prime Minister. Indeed some – including Purnell – were promoted after Blair had left.

But Purnell is part of a bigger, more portentous exodus, as MPs from across the political spectrum turn their backs on Parliament. Some of those departing were useless, but not all of them. Voters are fed up with politics. An ominous number of MPs, including Purnell, seem to share their disillusionment.