The assassination may have profound consequences both for the tattered foreign policy of President Bush and for those in the race to succeed him.

Mr Bush made a short and sombre statement at his ranch in Texas, condemning “this cowardly act by murderous extremists who are trying to undermine Pakistan’s democracy”.

Officials cut short their holidays to assess the impact on Pakistan, a pivotal ally in the fight against al-Qaeda. An intelligence report in July gave warning that its border region with Afghanistan had become a safe haven for terrorists.

Last week the US Congress imposed new conditions on military aid, requiring Pakistan to make “concerted efforts” to tackle the problem.

The Administration had invested much of its remaining diplomatic capital in attempting to broker reconciliation between Ms Bhutto and President Musharraf. Condoleezza Rice, the Secretary of State, persuaded Ms Bhutto to end her eight-year exile in October, while US officials put intense pressure on President Musharraf to resign as army chief and lift a state of emergency earlier this month.

Hopes that President Musharraf would offer a genuine power-sharing deal had begun to fade before yesterday. Sources said the White House would be watching to see if he goes ahead with the elections on January 8.

Yesterday, the White House insisted that any decisions about whether planned elections that Ms Bhutto’s party was expected to win on January 8 should go ahead were a matter for Pakistan’s government, but sources suggested that Mr Bush and his advisers would be watching closely to see if Mr Musharraf used the killing to justify a postponement.

There was also speculation, which the White House did little to dispel, that FBI officers could offer assistance in discovering who was behind the assassination. A spokesman for Mr Bush said yesterday that there were questions about why an investigation into a previous assassination attempt against Ms Bhutto had yielded so little information.

Hillary Clinton said that she was “profoundly saddened and outraged”. The Democrat presidential candidate added: “Her death is a terrible reminder of the work that remains to bring peace, stability and hope to regions of the globe too often paralysed by fear, hatred and violence.”

Her chief rival for the Democratic nomination, Barack Obama, said: “We will be steadfast in our desire to end the types of terrorist attacks that have blighted not just Pakistan but the rest of the world.”

The assassination, coming only a week before the Iowa caucuses, may switch voters’ attention away from domestic concerns, such as the economy or immigration, which had begun to dominate the debate.

Times Recommends

• A doctor, a daughter and a man called Johnny
• Mona Lisa's smile may belie ill health
• ‘Parasite economy’ lies behind Greek tragedy