BOSTON — Reminding Americans he has been tested in combat, John Kerry sought to ease doubts that he's tough enough to be commander in chief. He also tried to exploit President Bush's troubles with Iraq and raise questions about his administration's conduct.

Four years ago, Bush campaigned on a promise to restore "honor and dignity" to the Oval Office. In a turn on that theme, Kerry pledged to return "trust and credibility" to the White House.

He suggested that Vice President Dick Cheney had held secret meetings with polluters to rewrite environmental laws, that Defense Secretary Donald Rumsfeld had ignored the advice of military leaders and that Attorney General John Ashcroft had failed to uphold the Constitution.

In the most important speech of his long presidential campaign, Kerry decided to confront Bush head-on about Iraq, an issue the White House once had thought would win re-election for the president but instead has turned into a vulnerability.

"I will be a commander in chief who will never mislead us into war," Kerry said in one of the most pointed lines of his address, criticizing the president for asserting Iraq had weapons of mass destruction that have not been found.

Kerry's speech, while laced with uplifting themes, broke with three days of toned-down criticism of Bush.

"It strikes me as tough," said Thomas Mann, a political analyst at the Brookings Institution in Washington. "Frankly, not only does the convention want to hear it, but bottom line is, if Bush is to be defeated it's because of a judgment on his performance, and it looks to me as if Kerry is going after him on all counts."

On edge about terrorism, voters trust Bush over Kerry by a wide margin to keep the country safe. But Americans also are pessimistic about Iraq, where more than 900 U.S. soldiers have died, and Bush's justification for the invasion has been undercut by investigations in Congress and the Sept. 11 Commission.

"Saying we can fight a war on the cheap doesn't make it so," Kerry said. "And proclaiming 'Mission Accomplished' certainly doesn't make it so."

In a key passage of his speech, Kerry said:

"As president, I will ask hard questions and demand hard evidence. I will immediately reform the intelligence system so policy is guided by facts and facts are never distorted by politics. And as president, I will bring back this nation's time-honored tradition: The United States of America never goes to war because we want to. We go to war because we have to.

"I defended this country as a young man, and I will defend it as president. Let there be no mistake: I will never hesitate to use force when it is required."