The narrow vote to dismiss a resolution to block the transaction was a significant shift from a similar vote last year on a tank sale to Saudi Arabia under the Obama administration that was easily tabled, or killed, 71-27.
Both resolutions to disapprove of the sales were offered by Republican Sen. Rand Paul of Kentucky and Democratic Sen. Chris Murphy of Connecticut. But this time, Minority Leader Chuck Schumer, Foreign Relations ranking Democrat Ben Cardin of Maryland and most of the Democratic caucus joined their effort to oppose it.
Just five Democrats voted to support the sale -- Sens. Bill Nelson of Florida, Claire McCaskill of Missouri, Joe Manchin of West Virginia, Mark Warner of Virginia and Joe Donnelly of Indiana -- while four Republicans voted against the arms agreement.
The $510 million munitions sale is just a small slice of the $110 billion arms sale that President Donald Trump announced during his visit there last year. Most of that sale still has to be finalized, which means Congress will have a chance to vote on many future arms sales to Saudi Arabia, too.
"Regardless of whether the number (of votes) is 48 or 51 or 45, this is an important message to the Saudis that we are all watching," Murphy told reporters before the vote. "And if they continue to target civilians, if they continue to stop humanitarian aid from getting into Yemen, this vote will continue to go into the wrong direction for them."
Paul and Murphy argued that the US should not sell the precision-guided munitions to Saudi Arabia because of its role in the civil war in Yemen, where a Saudi-backed coalition has fought Iranian-backed Houthi rebels and has been accused of bombing civilians.
"We would stop this evil by sending a loud message to the President and a loud message to Saudi Arabia that we are not going to blindly support the arms race, we are not going to be blind to human rights transgressions," Paul said.
But senators backing the deal argued that the specific arms being sold to Saudi Arabia in this agreement would actually help them avoid civilian casualties.
"Saudi Arabia has already bought the bombs," said Sen. Bob Corker, the top Democrat on the Senate Foreign Relations Committee. "What we are selling them is the precision guidance systems that allow those bombs to be utilized in a way so you don't kill civilians.
"This is one of those things you're cutting your nose off to spite your face, and I think some are doing it because it's something Trump is proposing," Corker added.
Republicans argued that though Saudi Arabia does not have a great track record on human rights or terrorism, it remains an important ally in the fight against ISIS and efforts to counter Iran.
"The flaws of the Saudi government are real," said Sen. Lindsey Graham of South Carolina. "But my friends on the other side, particularly Sen. Paul, constantly put Saudi Arabia and Iran on the same footing. I think that is a very unwise analysis -- to suggest that Saudi Arabia is just as bad as Iran is just missing the point big time."
Paul and Murphy were able to force Tuesday's vote on the arms sale because of the arcane rules of the 1976 Arms Export Control Act, which gives any senator the ability to force a vote to disapprove of a foreign arms sale.
In addition to last year's vote on the sale of Saudi tanks, Paul forced a vote in March 2016 on the sale of F-16s to Pakistan, which was also defeated.