President Bush said Friday that Sen. Hillary Clinton, a potential candidate for the Democratic presidential nomination, is "formidable," but he declined to speculate on which Republicans might run for the White House in 2008.

"This is an unusual year because this is the first time there hasn't been a kind of natural successor in the party," Bush said in an interview with a national news network. "Two wide-open primaries with no sitting vice president running in either primary, so this is — I can't remember a time when it's been this open."

In a wide-ranging interview at the White House, Bush also took a hard-line stance against the Hamas party, which swept Palestinian elections on Wednesday. He said he'd emphasize the development of alternative fuels in his State of the Union address on Tuesday and shared his views on presidential powers.

On foreign issues, Bush said the United States would cut aid to the Palestinian government unless Hamas abolishes the militant arm of its party and stops calling for the destruction of Israel.

"If they don't, we won't deal with them," Bush said. "The aid packages won't go forward. That's their decision to make, but we won't be providing help to a government that wants to destroy our ally and friend."

He declined to predict whether the United States would still have large numbers of troops in Iraq when his successor takes office in 2009 but discussed the mistreatment of Iraqi prisoners at the Abu Graib prison.

"We were disgraced," he said.

"I know it caused a lot of people that want to like us to question whether they should, and equally important it gave the enemy an incredible propaganda tool — no question," Bush said.

Bush defended his order permitting the National Security Agency to listen in on phone calls and read e-mails of Americans suspected of communicating with terrorists. Critics claim the program violates civil liberties and say the government is circumventing the 1978 Foreign Intelligence Surveillance Act.

"I have looked at this program from all angles, and my dilemma and my problem is I can't explain to you how it works in order to justify your question without telling the enemy what we are doing," he said.

Asked if he thinks there is anything a president cannot do if he considers it necessary in an emergency, Bush said he thought there were "clear red lines" a president cannot cross.

"I don't think a president can ... order torture, for example," Bush said about his presidential powers under the Constitution. "I don't think a president can order the assassination of a leader of another country with which we're not at war."

On a personal note, Bush said that after he leaves office, he may be interested in setting up a think tank where young scholars could write and think about freedom and liberty. He also said he didn't think he'd have become president had he not married his partner of 28 years, Laura Bush.

"I don't know what I'd have been like if I hadn't married her," Bush said.