President Bush sought yesterday to assure the nation that the U.S. military is moving aggressively to defeat insurgents in Iraq, including the hooded men who decapitated American civilian Nicholas Berg, saying, "their barbarism cannot be appeased, and their hatred cannot be satisfied."

The message of Bush's weekly radio address was countered by Sen. John F. Kerry (D-Mass.), who said pointedly that U.S. troops sent into danger require "the right leadership" and "a clear idea of what they are -- and are not -- expected to do."

The dueling messages from the rivals for the White House in the November elections concluded a week in which news was dominated by video of Berg's gruesome death and ongoing furor in Washington over the abuse of Iraqi prisoners by U.S. military guards. Meanwhile, fresh polls suggest that public support for Bush has eroded to the lowest level of his presidency, with a majority now saying they oppose his handling of Iraq.

Against that tumultuous backdrop, Bush tried to use the death of Berg, a young businessman from Pennsylvania, as a rallying point for his policies in Iraq. "The savage execution of this innocent man reminds us of the true nature of our terrorist enemy, and the stakes in this struggle."

The president also elaborated on comments Friday by Secretary of State Colin L. Powell, who said that an interim Iraqi government, scheduled to assume power in six weeks, would possess the authority to ask U.S. troops to leave the country. U.S. officials said, however, they expected that the new government would want to retain the help of the nearly 170,000 troops while the climate there remains volatile.

Bush's radio speech emphasized that intention, saying that "the vital mission of our military in helping to provide security will continue on July 1st and beyond."

In Kerry's radio speech, on behalf of the Democratic Party, the senator and decorated Vietnam War veteran noted that yesterday was a holiday, Armed Forces Day. He did not mention Bush by name. But as he did earlier in the week, the presumed Democratic presidential nominee suggested that the administration was at fault for the abuse of Iraqis detained at Abu Ghraib prison outside Baghdad -- not a few wayward military guards, as Bush and his aides have said.

"We have a duty to guarantee that, when mistakes are made, those responsible are held accountable whether they are at the bottom of the chain of command or at the top," Kerry said.

Without saying that the Iraq war, for which he voted, was a mistake, Kerry reiterated his belief that the Bush administration has failed to obtain adequate help from other countries, develop strategies for a durable peace -- or create military norms to prevent the abuse of prisoners.

"[W]e have a duty to ensure that if our troops are sent into harm's way, they will have the right leadership, the right training, a clear sense of mission, a clear idea of what they are -- and are not -- expected to do," Kerry said.