President George W. Bush makes statement to reporters on opening day of Summit of the Americas

President Bush's chief political advisor remains under investigation by a special prosecutor looking into who revealed the name of a covert CIA officer. Questions about the fate of Karl Rove followed Mr. Bush to Argentina, where he is attending a hemispheric summit.

A week ago, Vice-President Cheney's chief of staff was indicted in the CIA leak case, and resigned. Karl Rove, the president's powerful senior aide, was not indicted, but remains under investigation.

The latest developments in the case dominated a brief session the president held with reporters here in Mar Del Plata, Friday. But he declined to be drawn into discussions of the matter. "You're trying to get me to comment on the investigation, which I'm not going to do. And I hope you understand that. It's a serious investigation, and it's an important investigation," he said.

Opposition Democrats have called on the president to fire Karl Rove.

Some members of Mr. Bush's own Republican party have urged staff changes in the White House in the wake of recent missteps. They include the slow response to aid victims of Hurricane Katrina and the revoked nomination of Harriet Miers to the Supreme Court.

All this, along with deepening public concern about the war in Iraq, has resulted in a steep decline in his ratings in recent opinion polls. But the president is making an all-out effort to convince the public that he is not distracted by the poll numbers, saying he intends to focus on a clear agenda, such as fighting terrrorism and boosting the U.S. economy. He says that is something that is important to all Americans, and that everyone can understand.

The leak case stemmed from a newspaper column that revealed the identity of CIA official Valerie Plame. Her husband, former Ambassador Joseph Wilson, has been a sharp critic of the president's Iraq policy. Prior to the invasion of Iraq, he was sent to Africa by the CIA to look into claims former Iraqi President Saddam Hussein was seeking materials for nuclear weapons. Mr. Wilson says he found no supporting evidence, and maintains an administration official illegally disclosed his wife's position at the CIA in an effort to discredit his findings, which the White House denies.