George Bush's comments came a day after Republicans on the Senate Committee on Thursday broke with the administration and approved a bill for detention and trial of foreign terrorism suspects.
Bush claims the measure would compromise the war on terrorism, saying "the enemy wants to attack us again".
Bush said he would work with Congress to resolve the disputed language, but stood firm on his demands.
"If not for this programme, our intelligence community believes al-Qaeda and its allies would have succeeded in launching another attack against the American homeland," he said.
"Unfortunately the recent Supreme Court decision put the future of this programme in question. We need this legislation to save it."
The high court earlier this year struck down Bush's current arrangement for trying detainees held at Guantanamo Bay, Cuba.
Bush said that it was vital to change the law to protect intelligence professionals who are called on to question detainees to obtain vital information. "They don't want to be tried as war criminals. They expect our government to give them clarity about what is right and what is wrong."
He called it an important debate that "defines whether or not we can protect ourselves. Congress has got a decision to make".
Meanwhile, foreign ministers of the European Union on Friday called on the US to respect international law in its handling of terror suspects after Bush acknowledged his country had run secret prisons abroad.
"We reiterate that in combatting terrorism, human rights and human standards have to be maintained," said Erkki Tuomioja, the Finnish foreign minister, speaking on behalf of the 25 EU ministers.
"We acknowledge the intention of the US administration to treat all detainees in accordance with the provisions of the Geneva Convention."
Four Republicans, led by former Republican presidential candidate, Senator John McCain, joined Democrats on Thursday on the Armed Services Committee and voted 15 to 9 for the measure that Bush opposes.
"As we work with the international community to defeat the terrorists and the extremists to provide an alternative to their hateful ideology, we must also provide our military and intelligence professionals with the tools they need to protect our country from another attack," Bush said.
"And the reason they need those tools is because the enemy wants to attack us again."
The dissident group led by McCain - and backed by Powell, Bush's first-term secretary of state - said Bush's approach would jeopardise the safety of US troops.
Warner (L) and McCain voted for
the measure that Bush opposes
Powell said: "Bush's proposal to redefine the Geneva Conventions would encourage the world to doubt the moral basis of our fight against terrorism and put our own troops at risk."
It was Bush's first news conference since August 21, when he said the Iraq war was "straining the psyche of our country" but that leaving now would be a disaster.
Bush has made the struggle against terrorism and the war in Iraq the top issues in the November elections, hoping to persuade voters that Republicans are better than Democrats at protecting the country.