Friday the Senate failed to re-authorize the USA Patriot Act, as a Democratic-led filibuster prevented a vote. This action — which leaves the act, key elements of which are due to expire on Dec. 31, in limbo — represents a grave potential threat to the nation's security.

I support the extension of the Patriot Act for one simple reason: Americans must use every legal and constitutional tool in their arsenal to fight terrorism and protect their lives and liberties.

The attacks of Sept. 11, 2001, made clear that the old rules no longer work. The terrorists who attacked us seek to kill innocent men, women and children of all races and creeds. They seek to destroy our liberties. They willingly kill themselves in their effort to bring death and suffering to as many innocents as they can, here in this country or anywhere in the world where freedom has a foothold.

In October 2001, after six weeks of intense scrutiny and debate, Congress overwhelmingly passed the Patriot Act (98 to 1 in the Senate and 356 to 66 in the House).

We had already received clear signals about our enemies' intentions, in the first attacks against the World Trade Center in 1993, the bombings of our embassies in Kenya and Tanzania in 1998 and the attack on the Navy destroyer Cole two years after that. Despite the abundance of warning signs, it took Sept. 11 to wake us to the dangers we face.

The central provision of the Patriot Act allows law enforcement and the intelligence community to share information. This might seem elementary, but for years law enforcement had been handcuffed by a legal wall that prevented agencies from the kind of inter-agency sharing that for four years now, thanks to the Patriot Act, has played an important role in preventing another day like Sept. 11.

These provisions were instrumental in investigations in Lackawanna, N.Y., and Portland, Ore., in which 12 people were ultimately convicted for attempts to aid al-Qaida and the Taliban.

So what happened in Washington? The House voted on Wednesday to renew the act; it stalled in the Senate. If the Senate fails to approve the extension, the government will be forced to revert in many ways to our pre-Sept. 11 methods. Sixteen provisions of the Patriot Act are set to expire on Dec. 31, including the key information-sharing ones.

It is simply false to claim, as some of its critics do, that this bill does not respond to concerns about civil liberties. The four-year extension of the Patriot Act, as passed by the House, would not only reauthorize the expiring provisions — allowing our Joint Terrorism Task Forces, National Counterterrorism Center and Terrorist Screening Center to continue their work uninterrupted — it would also make a number of common-sense clarifications and add dozens of additional civil liberties safeguards.

Concerns have been raised about the so-called library records provision; the bill adds safeguards. The same is true for roving wiretaps, "sneak and peek" searches, and access to counsel and courts, as well as many others concerns raised by groups like the American Library Association and the American Civil Liberties Union.

Given these improvements, there is simply no compelling argument for going backward in the fight against terrorism. Perhaps a reminder is in order.

The bipartisan 9/11 commission described a vivid example of how the old ways hurt us. In the summer of 2001, an FBI agent investigating two individuals we now know were hijackers on Sept. 11 asked to share information with another team of agents. This request was refused because of the "wall."

The agent's response was tragically prescient: "Someday, someone will die, and wall or not, the public will not understand why we were not more effective."

How quickly we forget.

Rudolph Giuliani was mayor of New York from 1994 through 2001. He wrote this commentary for The New York Times.