It was bad enough for Tony Blair to lose by 31 votes his first parliamentary vote yesterday since coming to office in 1997. It was much worse to lose it on an issue of trust over a core issue of national security - the alleged need to detain terrorist suspects for 90 days, which Mr Blair insists is required to defeat that threat
A majority of the House of Commons and 49 members of his own party decided not to believe him yesterday, recalling his failure of credibility over weapons of mass destruction in Iraq. This will inevitably affect his authority over contentious education, health and welfare reforms, the "radical third term agenda" which many Labour MPs say they will reject when these issues come up for decision next year.
Mr Blair's misjudgment of the political mood yesterday could, therefore, hasten his departure as prime minister. In the light of this vote, his announcement before the last election that he would not serve a full fourth term looks increasingly foolish. Such a stance opens up a lame duck scenario, even for the most powerful leaders. Psychologically, it invites speculation about alternatives and stimulates leadership challenges. Mr Blair could have anticipated a reduced majority in which he would have greater difficulty pursuing his new agenda without inviting such speculation as soon as he encountered political setbacks.
Mr Blair insisted last night that this was not a major political miscalculation, but a mistaken assessment by the parliamentary majority that the threat of terrorism requires a 28-day and not a 90-day detention period. He repeated his belief that police made a compelling case for the longer period, because of the threat posed by mass casualty terrorism to British citizens and last July's train bombings. Certainly this is a chilling prospect. But Mr Blair's ability to convince has been badly affected by the growing evidence that both parliament and public were misled over Iraq's possession of weapons of mass destruction.
This misjudgment invites a growing scepticism in other spheres - and inevitably an avalanche of comment on his demise. Mr Blair will need all his large stock of political capital to withstand it. In doing so, he must demonstrate conviction, insight and judgment in equal measure, along with authoritative leadership. He may find his base has eroded too much to sustain a reforming agenda, after the misjudgment which saw David Blunkett's departure from the cabinet last week and flurries of opposition over smoking bans and educational reforms. But he still enjoys popular support, compared to the main Conservative opposition which is in the middle of a leadership campaign.
Mr Blair still has lots of time before the next general election - although local ones next May will test those waters. But perceived weakening of authority invites opposition - and he is now more vulnerable to it. His party must decide whether it is better to rally round him as leader or face up sooner than expected to his departure.