Senator John McCain, a leader among Senate Republicans on military matters, called on Monday for the United States to lead airstrikes against Syria’s armed forces to protect the rebels and civilians there, much as it did in Libya last year.

He said the stakes were much higher in Syria, and that the United States should act without the approval of the United Nations, but with the support of Arab and other allies.

If the government of President Bashar al-Assad stays in power and continues the violent repression of dissent, “it would be a strategic and moral defeat for the United States,” Mr. McCain said on the Senate floor.

He said the crisis had reached “a decisive point” and made clear that the campaign he envisions would entail massive bombardment of the regime’s air defenses, which are extensive, even if its goal were limited to protecting the population from the army’s guns and providing some cover for humanitarian relief. He called as well for aiding the armed resistance militarily.

But he acknowledged that the United Nations Security Council, which authorized the Libya intervention, had been “shut down” by the opposition of China and Russia. “There will be no U.N. Security Council mandate,” he said, citing the 1999 intervention in Kosovo as a precedent for acting without the council’s formal approval. (The NATO allies acted then under cover of a previous resolution stemming from the breakup of Yugoslavia.)

The Pentagon’s initial reaction was unenthusiastic. Asked what Defense Secretary Leon Panetta thought of the speech, a senior official said: “The secretary is interested in exploring options that could help end the brutal violence in Syria, but he also recognizes that this is an extremely complex crisis. Intervention at this time could very well exacerbate problems inside the country.”

“Time is running out,” Mr. McCain said in an impassioned, half-hour-long speech to a mostly empty Senate chamber. “Assad’s forces are on the march. Providing military assistance to the Free Syrian Army and other opposition groups is necessary, but at this late hour, that alone will not be sufficient to stop the slaughter and save innocent lives. The only realistic way to do so is with foreign airpower.”

He continued: “At the request of the Syrian National Council, the Free Syrian Army, and Local Coordinating Committees inside the country, the United States should lead an international effort to protect key population centers in Syria, especially in the north, through airstrikes on Assad’s forces. To be clear: This will require the United States to suppress enemy air defenses in at least part of the country.

“The ultimate goal of airstrikes should be to establish and defend safe havens in Syria, especially in the north, in which opposition forces can organize and plan their political and military activities against Assad. These safe havens could serve as platforms for the delivery of humanitarian and military assistance – including weapons and ammunition, body armor and other personal protective equipment, tactical intelligence, secure communications equipment, food and water, and medical supplies. These safe havens could also help the Free Syrian Army and other armed groups in Syria to train and organize themselves into more cohesive and effective military forces, likely with the assistance of foreign partners.”

Mr. McCain, the ranking Republican on the Armed Services Committee and his party’s presidential candidate four years ago, was an early advocate of taking military action in Libya, where he called for imposing a no-flight zone to protect the population. This position enjoyed overwhelming support in the Senate. The Pentagon said that a no-flight zone would necessitate a forceful air campaign, not a mere decree backed up by patrols; by the time the United States and its allies intervened, Mr. McCain had become a vocal critic of the campaign, saying that its long delay had made it relatively ineffective.

Beyond the humanitarian and moral imperatives to act in Syria, he said, were core national security interests. He cited Syria’s close ties to Iran, its enmity toward Israel, its possession of chemical weapons and its past attempts to develop a nuclear capability.

He did not strongly condemn the Obama administration’s policy, endorsing its goal of removing the Assad regime, but said it was not at all apparent that it could succeed in the face of the regime’s determined attempts to extinguish the opposition.

“There is only one nation that can alter this dynamic, and that is the United States of America,” he said.

“Are there dangers, uncertainties and risks in this approach? Absolutely,” he said. But if the United States did not act, he said, “shame on us.”