SINGAPORE — From across the world, President Bush took on anti-war and anti-free trade Democrats who won control of Congress, saying Thursday any drift toward isolationism would hinder America's security and economic vitality.

"We hear voices calling for us to retreat from the world and close our doors to its opportunities," he said in a speech at the National University of Singapore. "These are the old temptations of isolationism and protectionism, and America must reject them."

Asserting that the spread of weapons of mass destruction to terrorists is "the greatest danger in our world today," Bush has the standoff over North Korea's nuclear weapons program atop the agenda in most of the meetings on his eight-day Asian trip. He urged allies to stand firm against a nuclear-armed North Korea — which he called "the most immediate threat of proliferation" in the region — and enforce U.N. sanctions against the country for test-firing a nuclear bomb last month.

"The transfer of nuclear weapons or material by North Korea to states or non-state entities would be considered a grave threat to the United States, and we would hold North Korea fully accountable for the consequences of such action," Bush said. "It is vital that the nations of this region send a message to North Korea that the proliferation of nuclear technology to hostile regimes or terrorist networks will not be tolerated."

With China's influence on the rise and his own stature weakened at home by last week's defeat of his Republican Party, Bush also sought to ease any doubt about the United States' long-term commitment to the region. He reassured nervous Asian allies that the United States will remain a reliable partner in liberalizing trade, confronting the spread of dangerous weapons and fighting terrorism, poverty and disease.

"We must maintain our presence in the Pacific. We must seize on our common opportunities. We must be willing to confront our common threats and we must help our partners build more hopeful societies in this vital part of the world."

The president's challenge to Democrats was clear but indirect. It came days after the House failed to pass legislation to normalize trade relations with Vietnam, a surprising setback before Bush's trip that was seen as a troubling sign for the future of his free-trade agenda once Democrats assume power.