Over the past week, President Bush and Vice President Cheney have thrown Sen. John F. Kerry on the defensive with a daily assault designed to tarnish his credentials as a possible commander in chief. But the orchestrated attacks also revealed the president's vulnerabilities on the issue that continues to shape the presidential campaign as much as any other.

The volleys over terrorism came after Kerry and his advisers believed they had put behind them most questions about his capacity to lead the country in a war on terrorism. Instead, Kerry and his advisers allowed themselves to be drawn into a new debate about Iraq and terrorism and were forced to rebut daily charges that Kerry has equivocated and sent conflicting signals on national security.

Kerry advisers see the criticisms as both wrong and distorted. But the exchanges are a reminder of how the issue of Iraq has bedeviled Kerry's candidacy first in the Democratic primaries and now the general election as he has navigated between the demands of the antiwar faction in his party and a desire to project strong leadership to a general-election audience.

The attacks also underscore the urgency within Bush's campaign to deny Kerry a sustained post-convention bounce. With some polls showing that Kerry made clear gains against Bush on terrorism and national security, the president's weakness on the issues that once were his great strengths is on clear display.

More than half the country disapproves of how the president has handled Iraq, and reservations about the situation there have spilled over into attitudes toward Bush on terrorism. The fighting there this week is a reminder that Iraq is far from stabilized, regardless of how much Bush talks about the progress that has been made. Given that reality, Bush has gone on the offensive against Kerry.

Kerry designed his convention in Boston around a single goal, to establish the Democratic nominee as capable of being commander in chief. He assembled his former Swift boat crewmates and retired military brass to offer testimonials to his courage, experience and judgment. Bolstered by some polls, Kerry advisers argued that the four-day convention did exactly what they had hoped.

But Kerry left himself susceptible to criticism with his effort to draw clear distinctions with Bush on how he would have dealt with Iraq before the war and how he would differ with Bush on the future course in Iraq and the war on terrorism. Bush and Cheney have seized on Kerry's comment that he would vote again to give Bush authority to go to war, his claim that he would try to reduce troop strength significantly during his first six months in office and his comment about waging a more sensitive war on terrorism.

The GOP attacks followed a familiar pattern. Bush struck first, elevating the issue and drawing more attention to the criticism than any of his surrogates could have attracted. Then Cheney moved in with tougher language designed to raise questions about Kerry's reliability. Bush and Cheney also selectively interpreted Kerry's words to cast them in the worst possible light.

At the beginning of the week, Kerry said that, even if he had known then what is known now about the failure to find weapons of mass destruction, he still would have voted to give Bush the authority to go to war. But he qualified that by criticizing Bush for going to war without more international support and for rushing to war without a plan to win the peace. "I would have done this very differently from the way President Bush has," he said. Bush chose to ignore that qualifier.

Cheney seized on a comment Kerry had made to the Unity convention of minority journalists about how he would differ from Bush on terrorism. "I believe I can fight a more effective, more thoughtful, more strategic, more proactive, more sensitive war on terror that reaches out to other nations and brings them to our side and lives up to American values in history," he said.

Cheney fired back that sensitivity never won a war. "America has been in too many wars for any of our wishes, but not a one of them was won by being sensitive," he said. "A 'sensitive war' will not destroy the evil men who killed 3,000 Americans and who seek the chemical, nuclear and biological weapons to kill hundreds of thousands more."

Kerry allies accuse the vice president of taking the comment out of context. Bush allies say it is Kerry who has sown confusion with his own words.

Bush has also put Kerry on the defensive over a comment the Democrat made about troop levels in Iraq. In an interview with National Public Radio, Kerry said, "I believe that within a year from now, we could significantly reduce American forces in Iraq, and that's my plan."

Bush responded that establishing artificial timetables for troop withdrawals will embolden insurgents in Iraq to wait out the United States and will make Iraqi citizens more timid in taking responsibility to defeat the insurgents themselves. Kerry advisers said Kerry was only setting a goal, not setting out a timetable. Troop reductions, they said, will depend on bringing more stability to Iraq and training more Iraqis as a security force.

But the Kerry advisers said the other key is to bring in troops from other countries to share the burden. They contended that Bush has so poisoned relationships that only a new president can succeed on that front. Some European diplomats and politicians have privately cast doubt on whether Kerry could easily do what Bush has not.

On the campaign trail, Bush devotes a significant portion of his stump speech to a vigorous defense of his actions in Iraq, which reflects his weakened position politically. But in defending himself as someone who has had no doubts about his decision to dislodge Saddam Hussein, he has contrasted himself with Kerry and has tried to cast doubt on his rival as an equivocator under whose leadership Hussein would still be in power.

In Phoenix on Tuesday night, Bush explained his decision to invade Iraq this way: "I had a choice to make. My choice was do I forget the lessons of September the 11th and hope for the best and trust the word and deeds of a madman, or do I take action to defend America. I will defend America every time."

Bush's goal appears aimed at shifting the focus of the debate from what has happened in Iraq to who can best be trusted to keep the country safe in the future, and he casts the choice as one between a president who knows the difference between good and evil and a challenger who finds shades of gray wherever he looks.

Kerry is equally determined to fight back on his own terms and to try to hold the president responsible for what has happened to the image of the United States around the world. It is a debate that will go a long way in determining the outcome of the election in November.

Sen. John F. Kerry has tried to dispel doubts about his ability to be commander in chief. But the Bush team has used Kerry's own words to raise new questions.