The US Senate has backed the Obama administration's plan to sell more than 1 billion dollars worth of American-made tanks and other weapons to Saudi Arabia.
The vote defeated a bid to derail the deal pushed by politicians critical of the kingdom's role in Yemen's civil war.
Senators who supported the sale said the US cannot deny its Middle East allies the weapons they need to combat Islamic State extremists and monitor Iran's aggression in the region.
"Blocking this sale of tanks will be interpreted by our Gulf partners, not just Saudi Arabia, as another sign that the United States of America is abandoning our commitment to the region and is an unreliable security partner," said Republican Senator John McCain, chairman of the Senate Armed Services Committee.
Although a resolution against the sale failed to advance on a vote of 71-27, the measure's sponsors said the debate demonstrated that congressional support for arms sales - even to a long-time and important Middle East ally - is not automatic.
They also insisted that Congress play a larger role in foreign policy decisions, especially those involving the use of military force.
The war in Yemen is pitting the country's internationally recognised government and a Saudi-led coalition against the Shiite rebels known as Houthis, who are allied with army units loyal to a former president.
The Saudi-led coalition has been carrying out air strikes in Yemen since March 2015 and thousands of civilians have been killed in the fighting, according to the UN human rights chief.
The US is supporting the Saudi-led coalition with intelligence and logistical support, including refuelling aircraft, according to senators opposed to the sale.
Most Americans are unaware of how involved their military is in Yemen, they said, adding that politicians have never fully discussed whether the participation advances US national security interests.
"Should Congress just lie down and be a lapdog for the president?" asked Republican Senator Rand Paul, a sponsor of the resolution. "Everyone should understand that this is a proxy vote for whether we should be at war in the Middle East."
Democratic Senator Chris Murphy, who also opposed the sale, said the Saudis have bombed areas that the US has asked them to avoid.
At the same time, IS and al Qaida are growing "by leaps and bounds," Mr Murphy said, because the Saudis are hitting primarily Houthi and civilian targets.
"Let's press the Saudis to get serious about spending more time as firefighters and less time as arsonists in the global fight against terrorism," Mr Murphy said.
The American Bar Association said last week there are credible reports Saudi forces have used American-made military equipment to carry out "indiscriminate and disproportionate" attacks on civilians.
The US should suspend further security assistance to Saudi Arabia "at least until such time as it can be credibly determined" that the allegations have been investigated and the kingdom is abiding by the law of armed conflict, the director of the bar association's governmental affairs office said in a September 14 letter to Mr Murphy and Mr Paul.
The Defence Department informed Congress of the proposed 1.15 billion dollar sale to Saudi Arabia on August 8.
The deal involves more than 100 main battle tanks, machine guns, smoke grenade launchers, night-vision devices, vehicles to recover damaged tanks from the battlefield, and thousands of rounds of training ammunition.
The primary contractor for the equipment is General Dynamics Land Systems of Sterling Heights, Michigan.
UN human rights chief Zeid Ra'ad al-Hussein last month called for an international investigation of rights abuses and violence in Yemen. His Geneva office released a 22-page report that chronicled abuses on both sides in the conflict.
Roughly 3,799 civilians have been killed since the air campaign began, according to the report.
The UN and rights groups estimated at least 9,000 people overall have died. Nearly three million more people have also been displaced inside the Arab world's poorest country.
Air strikes by the Saudi-led coalition were responsible for 60% of the 2,067 civilians killed in the conflict over a year-long span starting on July 1 2015, according to the report.
Just under a quarter - 475 - of the civilian deaths were attributed to rebel forces such as those loyal to former president Ali Abdullah Saleh, and 113 to affiliates of al Qaida and IS.