With the Senate delivering his first major defeat, President Bush leveled a stern warning at drug dealers, declaring "the hunting season is over" and those who kill cops should pay with their lives.

"The killing must stop," Bush declared Thursday in a photo-opportunity day trip to the Big Apple, where he climaxed a week of emphasis on the nation's drug war by speaking to federal narcotics agents and meeting with the widow of an undercover officer slain execution-style."We must repeat it until we are hoarse," he said, "repeat it until we are heard. To anyone who doesn't seem to understand what it is you are up against out there on the street: The killing must stop."

Bush's speech at the Drug Enforcement Administration's New York headquarters came as the Senate dealt him a historic defeat by voting down his nomination of John Tower as defense secretary. (Story on A1.)

The Tower controversy, however, prompted Vice President Dan Quayle, whose tie-breaking vote in the Senate might have been critical, to delay his own trip to New York to give the keynote address at a simultaneous, three-day Hemispheric Drug Conference.

"We've got to deliver some news to the bad guys," Bush told a room crowded with drug agents. "The hunting season is over. The rules on our side have changed, too, and we still need more change in those rules. But they're changing fast, and it's about time."

Bush used his speech to declare again his commitment to fighting illegal drugs and vowed to improve protection for law enforcement officials, who now often face semiautomatic weapons that he contends should not be banned.

"You know, it used to be unthinkable to shoot a cop. No longer," he acknowledged.