On Wednesday, the Senate defeated a bipartisan proposal to expand background checks for gun purchases, dealing a gut punch to President Obama and others who've pushed for tougher gun laws in the wake of last year's Sandy Hook massacre.

By a vote of 54-46, the Senate failed to reach the 60-vote threshold needed to advance the legislation. In doing so, the Senate shot down a proposal that had the support of 90 percent of the public, casting doubt on whether any new gun laws will emerge from Congress' most ambitious attempt in 20 years to rein in gun violence.

Four Republicans voted for the measure, while five Democrats voted against it.

The amendment was crafted by Sens. Joe Manchin (D-W.V.) and. Pat Toomey (R-Pa.), both of whom enjoy A ratings from the National Rifle Association. The proposal had become the centerpiece of gun control efforts as other proposals faltered. Given their proposal's bipartisan nature and its huge public appeal, it had widely been seen as the most likely amendment to make it into the broader gun bill the Senate is currently discussing.

Obama, in a fiery press conference after the vote, shamed Congress for failing to pass the measure, asking, "How can something have 90 percent support and still not happen?"

Furthermore, he specifically accused the NRA of using its enormous influence to frighten Senators into voting no. "Instead of supporting this compromise, the gun lobby and its allies willfully lied about the bill," he said. "This was a pretty shameful day for Washington."

"But this effort isn't over," he added. "I see this as just Round One."

The gun debate certainly is not over. But with the background check amendment off the table, the prospect of the broader gun bill clearing the Senate now appears even less likely.

The Senate this week is considering nine amendments to a base gun control bill, one of which is that background check provision. However, all of those amendments must garner 60 votes to advance, and with the votes expected to largely break down along party lines, the most contentious proposals have little to no chance of passing. Indeed, following the vote on background checks, the Senate also shot down two separate proposals to limit assault weapons and high-capacity magazines.

However, it was the background check amendment that was most crucial to securing passage of the final bill. As currently written, that bill contains a background check provision that's tougher than the Manchin-Toomey amendment, and as such it's not expected to pass.

"The vote jeopardizes the larger gun package, which also includes measures to increase penalties for gun traffickers and spending on school safety measures," says the Los Angeles Times' Michael A. Memoli. "The existing bill includes a more stringent background check plan that Manchin and others have said they do not support."

On Wednesday night, Majority Leader Harry Reid (D-Nev.) said he was pulling the entire gun bill from consideration, signaling that Democrats fear it's doomed.

There is still a chance the background check amendment could come up for a later vote. Reid, who supports the measure, voted no in a parliamentary move that will allow him bring it up for a vote in the future. That should give Manchin and Toomey more time to lobby for support.

Here's Bloomberg's Heidi Przybyla:

While Democratic leaders had accepted that the gun legislation may not pass the Senate, today’s series of votes on amendments might help lay the foundation for advancing in the future. Some senators could try to bring up a background-check proposal again, particularly if there is a backlash to the bill’s defeat. [Bloomberg]