John McCain and Lindsey O. Graham, both Republicans, represent Arizona and South Carolina in the Senate, respectively. Joseph I. Lieberman, an independent, represents Connecticut in the Senate.

As fighting inside Syria intensifies and the opposition there renews its plea to the world for help, the Obama administration’s hands-off approach is increasingly at odds with both America’s values and its interests.

Some have suggested that recent gains by the rebels — including offensives in Damascus and Aleppo, the assassination of key regime officials and several high-profile defections — prove that the Syrian opposition is on the path to victory and does not need our assistance.

Unfortunately, while opposition fighters inside the country have grown more capable in recent months, Bashar al-Assad’s regime is far from finished and is now unleashing even more indiscriminate violence against civilians, using tanks and artillery, helicopter gunships, militias, snipers and, for the first time, fighter aircraft.

Iran and Hezbollah are bolstering this assault with far-reaching material support because its leaders recognize that Assad’s fall would inflict a critical blow on them. Russia and China, meanwhile, continue to provide diplomatic cover for Assad’s brutality.

We are hopeful the rebels will ultimately prevail, but it remains a deeply unfair and brutal fight, and the speed and manner by which it is won matter enormously. All evidence suggests that, rather than peacefully surrendering power, Assad and his allies will fight to the bitter end, tearing apart the country in the process.

America’s disengagement from this conflict carries growing costs — for the Syrian people and for U.S. interests.

Because we have refused to provide the rebels the assistance that would tip the military balance decisively against Assad, the United States is increasingly seen across the Middle East as acquiescing to the continued slaughter of Arab and Muslim civilians. This reluctance to lead will, we fear — like our failure to stop the slaughter of the Kurds and Shiites under Saddam Hussein in Iraq or of the Tutsis in Rwanda — haunt our nation for years to come.

Our lack of active involvement on the ground in Syria also means that, when the Assad regime finally does fall, the Syrian people are likely to feel little goodwill toward the United States — in contrast to Libya, where profound gratitude for America’s help in the war against Moammar Gaddafi has laid the foundation for a bright new chapter in relations between our two countries.

Much more than in Libya, moreover, the United States has significant national security interests at stake in Syria. These include preventing the use or transfer of the regime’s massive chemical- and biological-weapons stockpiles — a real and growing danger — and ensuring that al-Qaeda and its violent brethren are unable to secure a new foothold in the heart of the Middle East. Our decisions and actions have been woefully insufficient to safeguard these interests and others.

The U.S. reluctance to intervene in Syria is, first of all, allowing this conflict to be longer and bloodier, a radicalizing dynamic. Contrary to critics who argue that a greater U.S. role in Syria could empower al-Qaeda, it is the lack of strong U.S. assistance to responsible fighters inside the country that is ceding the field to extremists there.

It is not too late for the United States to shift course. First, we can and should directly and openly provide robust assistance to the armed opposition, including weapons, intelligence and training. Whatever the risks of our doing so, they are far outweighed by the risks of continuing to sit on our hands, hoping for the best.

American help should go to those groups that reject extremism and sectarianism in both word and deed. As in Libya, the relationships we build with armed groups inside Syria now will be indispensable going forward.

Second, since the rebels have increasingly established de facto safe zones in parts of Syria, the United States should work with our allies to reinforce those areas, as Secretary of State Hillary Rodham Clinton suggested last week. This would not require any U.S. troops on the ground but could involve limited use of our airpower and other unique U.S. assets.

We know there are risks associated with deepening our involvement in the profoundly complex and vicious conflict in Syria. But inaction carries even greater risks for the United States — in lives lost, strategic opportunities squandered and values compromised. By continuing to sit on the sidelines of a battle that will help determine the future of the Middle East, we are jeopardizing both our national security interests and our moral standing in the world.