By-elections have an honourable place in British political history. For Mar-garet Thatcher it was the loss of the safe Tory seats of Mid-Staffs and East-bourne that confirmed she had become an electoral liability. For Gordon Brown it has taken the London mayoral battle, Crewe & Nantwich and now Glasgow East to prove that the prime minister is an election loser. And this weekend his party knows it.

Mr Brown says he will “get on with the job” and that voters are just venting their anger over the economy. He thinks the storm will pass, although his own chancellor has warned of a long and “profound” downturn and forecasters are unanimous in predicting that next year will be even worse. Mr Brown says that he alone can expose the inexperience and shallowness of the Tories.

For his cabinet colleagues, and every Labour MP who has calculated what Glasgow East’s 22% swing would do to their majority, the question is whether they believe him. They know the economic news will get worse and unemployment will soar; in challenging times they need a leader who can reassure and inspire.

They also know that in comparison with David Cameron, and even more Barack Obama, who cruised through Downing Street yesterday, the prime minister looks old, dour and leaden-footed. Nicolas Sarkozy is only four years younger than Mr Brown yet looks part of the new generation emerging on the world stage. Yes, MPs say, but old age and cunning will always outwit youth and enthusiasm. This is sadly not true in Mr Brown’s case. Despite a decade in power and a lifetime in politics, he has shown himself to be tactically inept.

He made three key errors from the outset. The first was his failure to have a leadership contest to prove that he could fight and win. The second was the failure to commit to an immediate general election to endorse his leadership and the third was the failure to call that election when he could have won and secured a fresh five-year term just as the economy was turning.

In just one year as prime minister, Mr Brown has reached the point that it took Mrs Thatcher 11 years to get to. The electorate have had a good look and decided they do not like what they see. When voters conclude that a leader has passed the point of no return, there is no way back. That presents a painful dilemma to the party. It could do nothing, which is the easy way out. But that almost certainly will mean electoral oblivion in 2010 and many years in the wilderness.

Or it could act, urging Mr Brown to stand down or face a leadership challenge. A Labour contest this autumn would allow the party’s rising stars to outline their visions for the future. As the Tories discovered in 2005, contests can have surprising consequences. Not only did Mr Cameron emerge but so did a reinvigorated party.

A leadership battle would have to be followed by a general election – there is no precedent for two unelected prime ministers in succession – but that might not be such a bad thing. A new leader would enjoy a political honeymoon and Mr Cameron would no longer be the only fresh face in town. True, Labour might still lose but it would have a better chance of limiting the rout. And if the economy is as bad as analysts seem to think, the Tories would come to power in the midst of a recession. They might even, in a delicious irony, have to raise taxes or cut public services. The chances of a Labour bounce back at the next election would be much greater.

The prospect of the government lurching from crisis to crisis, desperate for eye-catching initiatives to lift itself in the polls, is almost too depressing to contemplate. As Mr Obama tells us about America, the time for change is here.