Meanwhile, four Republicans also crossed the aisle in the other direction to vote for background checks. Those were Sens. Pat Toomey (PA), Mark Kirk (IL), Susan Collins (ME) and John McCain (AZ).
Here's a rundown of the four Democratic defectors and their reasons.
Max Baucus (MT)
Baucus, the chairman of the powerful Finance Committee, is up for reelection next year and carefully watching his back against conservative challengers. He's been tight-lipped on the issue, repeatedly saying he'll reflect the wishes of the residents of his small, largely rural state. Asked to explain his vote, Baucus was terse.
"Montana," he said.
Mark Begich (AK)
Begich, a first-term senator, faces reelection next year in bright-red Alaska. Extremely cautious not to be seen as sympathetic in the least to gun control supporters, he voted against opening debate on gun legislation, even as many Republicans voted for debate. Begich characterized the issue as one between emotions and sound judgment.
"It's dangerous to do any type of policy in an emotional moment," the senator said. "Because human emotions then drive the decision. Everyone's all worked up. That's not enough."
Mark Pryor (AR)
The low-profile Pryor, also up for reelection in the highly conservative state of Arkansas, is a primary target for Republicans seeking to peel off a Democratic senator. On the issue of guns, Pryor has signaled no solidarity with progressives.
"I oppose the Manchin-Toomey amendment," he said before the vote. "Instead, I will support the Grassley amendment that enforces the laws we already have on the books. As I've said before, I believe we need to deal with the real issues behind gun violence, and that includes mental health and school safety. The Grassley amendment accomplishes this while protecting the rights of law-abiding Arkansans."
Heidi Heitkamp (ND)
The newly elected freshman squeaked out a victory last November in her small, deeply conservative state by fewer than 3,000 votes.
"I've been adamant from the very beginning of this conversation that the focus should be on mental health issues, full and accurate reporting into the [National Instant Criminal Background Check System] database and ensuring that we are prosecuting criminals in possession of or trying to possess firearms," she said in a statement before the vote. "This conversation should be about what is in people's minds, not about what is in their hands. I commend Senators Manchin and Toomey for working so hard to bring a serious bill to the floor. However, in its current form I do not see a path for my support. I've thought long and hard about this, I've taken the tough meetings, and I've heard overwhelmingly from the people of North Dakota; and at the end of the day my duty is to listen to and represent the people of North Dakota."
(Note: Senate Majority Leader Harry Reid (D-NV) supported the legislation but switched his vote to no in order to reserve the right to bring up the bill for consideration later. It fell 54-46.)