George W Bush faces 90 of the most critical minutes of his presidency, with his rationale for war in Iraq in tatters and his bid for a second term threatened by challenger John Kerry.

Bush, who shepherded the United States through the agony of September 11 and led the country into two wars in Afghanistan and Iraq, finds his authority challenged as never before going into the second of three televised presidential debates. After scowling through a lacklustre showing in the first head-to-head with Kerry in Florida last week, Bush is under pressure to check the Democrat’s opinion poll surge ahead of a third head-to-head with Kerry next week.

But aides to Massachusetts’s senator Kerry expect a reinvigorated Bush to emerge in the second debate.

“The wild card here is which President will show up, will it be the disinterested, annoyed and unprepared one ... or will it be the one we expect,” said Kerry campaign strategist Joe Lockhart. But Republicans are itching to put Kerry on the spot for what they say is a long catalogue of inconsistency on how best to defend the United States. The debate is “an opportunity for Senator Kerry to defend a 30 year record of being wrong on defence,” said Bush campaign manager Ken Mehlman.

Friday’s showdown is promising to be one of the most bruising encounters in recent memory, with Bush branding Kerry’s foreign policy a danger to America, and his challenger accusing him of weaving a “pattern of deception” over Iraq. The argument has sharpened in the week and one day since the first debate, with Kerry sniffing blood and the Republicans realising that an expected coast to re-election may not materialise.

Both candidates will take questions from audience members, in a “town hall “ style-debate supposed to mix foreign policy with economic questions.

That format supposedly will play into Bush’s common touch, although Vietnam War veteran Kerry, seen as aloof at the beginning of his campaign, has honed his technique in a blizzard of town hall meetings in recent weeks. The war in Iraq, which emerged slowly as a campaign issue but has become a prism through which each candidate’s character is viewed, will likely dominate the debate.

Bush is dealing with a week of bad news on Iraq. Former top US official in Baghdad Paul Bremer was this week caught complaining he never had enough troops to stabilise the country after the war, although he endorsed on Friday Bush’s decision to topple the government of Saddam Hussein.

And a report of the Iraq Survey Group set up to probe Saddam Hussein’s alleged weapons of mass destruction programmes, said the Iraqi dictator had no such arms at the time of the US invasion in 2003, undercutting Bush’s rationale for war.

But Bush has been underestimated before, and a bravura performance could shift the race, which has been seesawing between himself and Kerry for months in his favour. Republicans say Bush will use the encounter to expose Kerry’s perceived contradictions on foreign policy and how to deal with Iraq.

But Kerry aides are taking aim at what they see as a fantasyland approach to a deepening insurgency in Iraq.

“The vice president said he would not change a thing, the president said he would not change a thing, that is something they have to defend tomorrow night,” senior Kerry aide Joe Lockhart told reporters.

Bush will try to dispel the unflattering images of the first debate, where he appeared in cutaway camera shots irritated with Kerry’s critique of his stewardship of the Iraq war. Interestingly, his situation mirrors that of his opponent in 2000, then vice president Al Gore, whose second debate performance was hamstrung by a need to dispel images of his showing in the first encounter, compromised by his theatrical sighs. Both Kerry and Bush marked out their battle lines Thursday before the debate.

Bush declared he would attack Iraq all over again, shrugging off the report that Baghdad had lacked the unconventional weapons at the heart of his case for war.

“Based on all the information we have to date, I believe we were right to take action, and America is safer today with Saddam Hussein in prison,” Bush said in a hastily announced prepared statement as he departed the White House.

But Kerry said Bush was being dishonest with the American people.

“President Bush’s serious errors in judgement have left us more vulnerable and less safe as the terrorists continue to murder school children and target our brave soldiers,” he said as he prepared for the debate in Colorado.