Democrat Sen. Joe Manchin and Republican Sen. Pat Toomey worked to create a bipartisan background check bill

UPDATED: Background checks failed to get the 60 votes required to pass in the Senate. The final vote was Yes: 54 No: 46 Sen. Harry Reid changed his vote to No at the last minute in order to reserve his right to bring it back to the floor. 
Republican Sens. Pat Toomey (PA), Mark Kirk (IL), Susan Collins (ME) and John McCain (AZ) voted Yes. Democratic Sens. Heidi Heitkamp (ND), Max Baucus (MT), Mark Begich (AK) and Mark Pryor (AR)voted against background checks.
Even though polls indicate it's supported by the vast majority of Americans, the gun-control proposal with arguably the best chance of Senate passage may very well go down to defeat on Wednesday.

The Senate is scheduled to begin voting Wednesday afternoon on a number of proposals to reduce gun violence, including a bipartisan yet controversial agreement on expanding background checks proposed by Sen. Joe Manchin, D-West Virginia, and Sen. Pat Toomey, R-Pennsylvania. Their proposal would extend current background checks for gun buyers to include gun shows and internet sales.

While Democratic leaders expressed optimism they'd get enough votes to pass the Manchin-Toomey amendment, a CNN tally of senators indicates the measure is probably in major trouble of passing, unless several undecided Democrats and Republicans -- mostly from conservative states -- throw their support behind the amendment.

Just about every national poll conducted since December's horrific shootings by a gunman at a Connecticut elementary school, which left 20 young students and 6 adults dead, has indicated widespread support for increased background checks. That includes a CNN/ORC International survey released last week that indicated 86 percent of the public supports some form of background checks that are not currently required by law for gun sales. And 86 percent of Americans questioned in an ABC News/Washington Post poll released Tuesday said they supported background checks for gun sales on the internet and at gun shows.

The two new polls were also in-line with past surveys by indicating no partisan divide on the question, with the vast majority of Democrats, independents, and even Republicans supporting increased background checks. The ABC/Washington Post survey also indicates that 86 percent of gun owning households support the proposal.

President Barack Obama's been a vocal advocate for passing gun control legislation, and he's touted public opinion as he pushes Congress to act.

"If our democracy's working the way it's supposed to, and 90 percent agree on something, in the wake of a tragedy, you would think this would not be a heavy lift," Obama said last week.

But the powerful National Rifle Association, the leading advocate on gun rights, fiercely opposes the Manchin-Toomey compromise. And that influential opposition is a counterweight to public opinion.

And the ABC/Washington Post poll also highlights that activism and engagement may be a factor in this political battle. About one in five gun owners questioned in the survey say they have at some point contacted a public official to express their views on gun control. That number drops by half for those in non-gun households. Nineteen percent of gun owners say they've contributed to an organization engaged in the gun control issue, with just 4 percent of non-gun owners saying the same thing.

Another factor may be public concerns that increased background checks would lead to a federal registry of gun owners and their firearms, which according to the CNN/ORC poll, is opposed by 55 percent of Americans. And two-thirds of those questioned in survey said that if the government did keep a list of gun owners, it would eventually use that list to take guns away from people who own them.

To allay such concerns, the Manchin-Toomey proposal includes language to bar the creation of such a federal registry. Manchin, Wednesday on the floor of the Senate, said attempts by gun control opponents to portray his bill as a universal background check measure were lies.

CNN Senior Congressional Producer Ted Barrett contributed to this report