The Manchin-Toomey bill, which would extend background checks to gun shows and online sales, seemed like gun-control advocates' best bet. It was co-authored by Pat Toomey, a Republican with an "A" rating from the NRA. The second-biggest gun rights group in the country initially endorsed it as a "pro-gun bill." (They have since pulled their support).

But on Wednesday, Sen. Joe Manchin (D-W.V.) declared his own bill dead:

Why won't the bill get the 60 votes it needs to survive?

Several Republicans who Toomey and Manchin were counting on bailed, including Sen. Kelly Ayotte (R-N.H.), who, according to The Washington Post, said the bill "would place unnecessary burdens on law-abiding gun owners and allow for potential overreach by the federal government into private gun sales." Sen. Lisa Murkowski (R-Alaska) also decided to vote no, even after Toomey and Manchin proposed background check exceptions for rural gun buyers who live far away from a licensed dealer, reports the Pittsburgh-Post Gazette.

With an immigration battle coming up, some conservative senators — like Marco Rubio — were probably scared of voting with Democrats on two important issues, Manchin theorized to NBC. Then there were the five Democratic senators from conservative states who were hesitant to support the bill, especially the ones up for re-election in 2014, reports TIME.

Murkowski and around 20 Republican senators were also lured away from the Manchin-Toomey bill by less restrictive legislation written by Sen. Chuck Grassley (R-Iowa). That bill focuses mainly on preventing gun-trafficking, creating mental health databases, and giving funds to schools for increased security.

Part of the problem liberals face when it comes to gun control in the Senate is that the odds are inherently stacked against them, writes Jonathan Chait at New York:

The Senate gives vastly disproportionate representation to small states, which are mostly rural (that’s why they're small) and thus much more pro-gun … Even if you win a majority in the Senate, it isn’t enough. You need 60 votes to break a filibuster. When you combine [them], you hand a small rural minority overwhelming power. [New York Magazine]

What about those polls Democrats love to cite that say 90 percent of Americans support background checks?

"Gun control proposals poll decently all the time," writes Mother Jones' Kevin Drum. "But the plain truth is that there are only a small number of people who feel really strongly about it, and they mostly live in urban blue districts already. Outside of that, pro-gun control opinion is about an inch deep."

That lack of support is also expected to kill bans on assault weapons and high-capacity magazines, which are also up for a vote on Wednesday.