There's a tangibility to numbers that's hard to escape in the gun control debate. Twenty-six killed at Newtown. 3,300 dead from gun violence since. A (debatable) 40 percent of gun sales that currently require no background check. But this week, the most important number is 60: the number of senators needed to support a compromise bill on background checks in order for it to pass.

The compromise on background checks, proposed by Sens. Joe Manchin of West Virginia, a Democrat, and Pat Toomey of Pennsylvania, a Republican, is the most contentious element of the gun package. Or, rather, it will be — it isn't yet included in the package legislation, which currently includes a far more sweeping bill introduced by Sen. Chuck Schumer of New York. The compromise, which would require background checks for sales at gun shows and for some private sales, will join measures increasing penalties for gun trafficking and funding for school safety.

If the compromise makes it into the package at all. America has come to terms with the current political reality that controversial legislation in the Senate requires 60 votes to pass. Without 60 votes, opponents of legislation can quietly "filibuster" a bill indefinitely. The new math is not even qualified in The Times article on the debate ("push the bill toward the 60 votes needed for final passage") or the one in the Washington Post (which says the assault weapons ban is "expected to fall far short of the 60 votes needed to ensure final passage") — it's just how it is. In the case of the background check compromise, a filibuster is all but certain; the NRA has pledged to include the vote on the filibuster in its grading system for elected officials. Anyone wanting an A+ grade from the NRA better ace the math.

Proponents of the bill are clear that the vote will be tight. Sen. Chris Murphy of Connecticut told Morning Joe today that if the measure passes, it will be with "60, 61 votes." Manchin and Toomey are stalking their colleagues to get support. As The Times reports:

Mr. Manchin, who spent much of last week buttonholing colleagues at the Senate gym and giving out handwritten pleas for support on the Senate floor, said he felt certain that people who read the bill would find their objections quelled. “The thing is, the easiest vote for me or any senator to make is ‘no,’ ” he said.

Which means that Capitol Hill will spend the beginning of the week engaged in its favorite game: wooing the wavering. A few new positions were revealed through week's initial vote on a filibuster that threatened to keep the package of new legislation from reaching a vote at all. Sen. Mark Pryor of Arkansas, a Democrat who faces an uphill reelection fight next year, voted against "ending debate" — in other words, against the package — indicating that he's likely to oppose the bill when it reaches the floor this week. Sen. Mark Begich of Alaska did the same. While the Post reports that "the bill will need the backing of at least six Republican senators to pass," defections among Democrats make that figure seem low. (There are 45 Republicans in the Senate, 53 Democrats, and two independents that tend to vote with the Democrats.) This weekend, Sen. Susan Collins of Maine, a Republican, indicated that she would support the background check compromise — only the third Republican to do so.

It is not only Senators Manchin and Toomey who will be twisting arms. New York mayor Michael Bloomberg's gun control group Mayors Against Illegal Guns continues to pressure senators in their home states, this weekend unveiling new ads targeting Arizona's Jeff Flake.

It's an interesting strategy that focuses in part on the shifting dynamics of gun legislation. As The Atlantic Cities reported last week, suburban areas are increasingly the battleground over gun politics, with urban residents largely favoring new controls and rural areas opposing them. Flake's Arizona is home to some of the fastest-growing suburbs in the United States.

There is also a small fissure among gun rights groups. Over the weekend, the Citizens Committee for the Right to Keep and Bear Arms endorsed the background check compromise. While the group is far smaller than the NRA — and likely lacks the political clout of Gun Owners of America — it allows those pushing for the compromise to suggest that the matter isn't cut and dried for gun rights advocates.

If the Senate does manage to put together its 60 or 61 votes and pass the bill, attention will turn to a new number: 218. That's what it takes to for legislation to pass the House of Representatives. And 218 is a much, much higher number than 60, in more ways than one.