The US Senate today turned away a bipartisan proposal to expand background checks for the sale of guns, signaling a huge defeat for President Obama's administration and Congressional Democrats. The 54-46 vote fell short of the 60 votes needed to pass the measure, due to a GOP-led filibuster. But the amendment was hurt even more by the president's own party; four Democratic senators, who could have carried the bill to passage, rejected the background check amendment: Mark Begich (D, AK), Max Baucus (D, MT), Heidi Heitkamp (D, ND), and Mark Pryor (D, AR). Republicans in favor of the background check amendment included Pat Toomey (R, PA), Susan Collins (R, MA), Mark Kirk (R, IL), and John McCain (R, AZ).

As The Washington Post reports, support for expanded background checks looks very different outside the halls of the Senate; a recent Washington Post-ABC News poll found that 9 in 10 Americans favor strengthened background checks, with strong support even among NRA members and gun-owning households.

The failure is particularly biting for many in light of the dramatic gun violence from last December, when 20 children and six adults were murdered in Newtown, Connecticut. Despite the broad sense of national consensus that followed the Newtown tragedy, it appears that the incident did not actually change anything about gun politics in Congress.

Critics of the Senate's failure to act cite influence from special interests, namely the NRA, which has stepped up its marketing efforts in recent months as tragedies in Connecticut, Colorado, and other areas have thrust gun control into the national spotlight. As part of its outreach efforts, the NRA won a sponsorship for NASCAR which renamed the Samsung 500 to the NRA 500 this April. It's also wielded strong language; the NRA accused the president of being "just another elitist hypocrite" for accepting Secret Service protection while turning down its proposal to put armed guards in schools. As The Washington Post reports, the NRA launched a call and email campaign today to oppose the legislation, as well as a $500,000 internet ad buy.

In a statement released following the amendment's defeat, New York City Mayor Michael Bloomberg, co-chair of Mayors Against Illegal Guns, said that "today's vote is a damning indictment of the stranglehold that special interests have on Washington. More than 40 US Senators would rather turn their backs on the 90 percent of Americans who support comprehensive background checks than buck the increasingly extremist wing of the gun lobby."

As Reuters reported earlier this year, the current bill under consideration is part of the biggest effort to control guns in the country in decades. The administration has sought to ban assault weapons and institute background checks for all buyers, which are intended to screen out felons and those with mental illnesses. It's not clear where efforts to secure broader background checks will go from here, but President Obama will speak at 5:30 ET today on gun control after the Manchin-Toomey compromise amendment on background checks failed to pass today.

Update: Multiple sources are reporting that the US Senate also voted down a federal assault weapons ban, by a 40-60 vote.

Update: President Obama just finished a scathing speech (full transcript) in which he rebuked Congress for failing to act on "common sense" background check measures. "I'm gonna speak plainly and honestly about what's happened here, because the American people are trying to figure it out," Obama said. "The gun lobby and its allies willfully lied about the bill."

The president admonished senators for rejecting the measure, noting that a small minority of gun advocates had intimidated them and influenced their vote. "There were no coherent arguments as why we wouldn't do this," the president said. "They caved to the pressure."

President Obama encouraged further action on gun control, and suggested the legislative battle will continue. "If this Congress refuses to listen to the American people and pass common sense gun legislation, then the real impact is going to have to come from the voters," Obama said. "Those who care deeply about preventing more and more gun violence will have to be as passionate and as organized and as vocal as those who blocked these common sense steps to help keep our kids safe."

"Everybody talked about how we were going to change something [after Newtown]," President Obama said. "Sooner or later, we are gonna get this right."

Update: An amendment vote to ban the sale of high-capacity magazines has also been defeated in a 46-54 vote.