Mark Kelly and Gabrielle Giffords sent out a bulletin this morning urging people to contact their senators to urge them to support the Toomey-Manchin compromise on background checks. The couple acknowledged that there is not enough support to break a filibuster on a vote scheduled for later today :

We won't waste any time getting to the point.

The U.S. Senate is set to vote on expanding background checks and other legislation to reduce gun violence today at 4pm Eastern.

The truth is that as of right now, it's very close, but we don't have the votes. That's why it's so critical you make your voice heard in the next few hours.

Slate rounds up the latest on the gun bill here.

New York magazine's Jonathan Chait has six reasons why the background-check compromise is on the rocks. Reasons 1-3:

1. Getting a majority of America isn’t the same thing as getting a majority in Congress, because Congress doesn’t perfectly represent America. The House represents a collection of districts, the overwhelming majority of which are not competitive and produced a nearly impervious Republican majority despite Democrats receiving more votes in aggregate. 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.

2. Even if you win a majority in the Senate, it isn’t enough. You need 60 votes to break a filibuster. When you combine 1 with 2, you hand a small rural minority overwhelming power.

3. Almost everybody may support background checks, but not every American knows the actual content of every bill that gets a vote. Look at health-care reform. Americans overwhelmingly favor nearly every provision of Obamacare, but oppose the law because they had a general sense of not liking it.

Likewise, opponents have turned the debate into a general discussion of “gun control,” which is way less popular than a specific law about background checks. Lisa Murkowski explains her No vote thusly: “In Alaska you're pretty much pro-gun. That about sums it up.”