From a mechanical standpoint, the news is unremarkable: Senators from West Virginia and Pennsylvania have reached a tentative agreement on a policy measure. From a political standpoint, though, it's enormous — a new deal on background checks leaps a critical hurdle in the president's push for a package of new gun restrictions.

Senators Joe Manchin and Pat Toomey were reported to be working on a deal on the issue earlier this week. They were only the latest group to tackle the issue, on for which it's hard to overstate how crucial a compromise could be. After the massacre at Sandy Hook, the president made increased background checks a critical component of his response. Despite being an enormously popular reform, a deal between Sen. Schumer of New York and Tom Coburn of Oklahoma fell through over Coburn's concerns about record-keeping (and due to ferocious lobbying from gun advocates).

So Senate Majority Leader Harry Reid pushed out a vote on the full package of reforms — which also includes increased funding for school safety and new restrictions on trafficking — to give senators more time to reach a deal. Manchin and Toomey were the last hope for a proposal that was amenable to both Democrats and Republicans, preserving the possibility that the entire package could be approved in the Senate. With a new deadline set by Reid, this morning it appears that Manchin and Toomey have been successful.

The Huffington Post reports on what the deal, which will be announced this morning at 11 a.m., is likely to include.

The bill would expand background checks to all commercial gun transactions, including those that take place at gun shows, according to a source close to negotiations. It would exempt family transfers and gifts or temporary transfers for hunters and sportsmen, though the exact language was still being negotiated. There will be record-keeping for transactions in which a background check takes place, though it remained unclear who would keep those records. The expanded background checks would be done through a federal firearm license holder instead of through an online portal — a policy win for gun control advocates.

Those paying close attention to the debate will note an area of concern: that remaining uncertainty about record-keeping. A primary objection to an expansion of background checks has been concern that government maintenance of such records, while easier for casual sellers, could lead to a kind of national registry for gun owners. Such a registry is illegal under the Firearm Owners Protection Act.

Without bipartisan agreement on background checks, the already shaky gun package would almost certainly not pass the Senate. Even with that agreement, the legislation will face a difficult vote. A group of nearly a dozen Republican senators has pledged to filibuster the deal regardless of its contents, although that pledge seemed to be fizzling Tuesday afternoon and into the night. An editorial in the Wall Street Journal today calls for the filibuster to be scuttled, suggesting that if "conservatives want to prove their gun-control bona fides, the way to do it is to debate the merits and vote on the floor." It seems likely that there will more than 60 senators willing to keep a filibuster from blocking a vote, anyway — but there's no guarantee it will then pass.

How critical is a background checks to that passage? In an interview for CNN's Anderson Cooper 360, former Rep. Gabby Giffords and her husband Mark Kelly discussed gun politics.

At the 3:20 mark, CNN's Dana Bash asks, "If you were to name the number one thing Congress could do to prevent the kind of violence that you were the victim of, what would it be?" Kelly turns to Giffords; she to him. Looking him in the eye, she nods and says, "Background checks."

That's probably not the number one thing Congress could do, but it's certainly one of the more realistic possibilities. And as Giffords and Kelly certainly know, it's the issue on which it's most critical they apply pressure at this moment. They won't be alone in doing so: Families from Newtown, Connecticut, will join hundreds of others in reading aloud the names of those killed by gun violence prior to Thursday's vote.

If there is a tenable deal, and if there's no filibuster, and if the Senate approves it, President Obama and advocates for reform will have reached the top of the hill. And from that vantage, they'll see the mountain: the House of Representatives, where even a brokered compromise on gun measures seems like high aspiration.

But one climb at a time.