As is usually the case in DC, we can predict the outcome of the Senate's vote on a unified package of gun measures a few weeks before the vote happens. One of the only question marks — universal background checks — may also be resolved, thanks to a big name. Not Bloomberg. McCain.

What the Senate will vote on next month includes three elements: a measure to increase penalties for illegal gun sales, increased funding for school protection, and universal background checks. A measure to ban assault weapons will be voted on as an amendment to the main package. The first two components passed the Senate Judiciary Committee with bipartisan support; the second two, on a party-line vote.

It's pretty clear how three of the four will fare. The fourth may come down to the senior senator from Arizona.

Likely to pass:
Both the measure to curb gun trafficking and the school security funding bill are likely to pass. (Background on all the components can be found here.)

The trafficking bill, introduced by Sen. Leahy, broadens the definition of an illegal gun sale — for example, buying a gun for someone blocked from doing so and then selling it to them — and increases penalties for such crimes. Even the NRA's Wayne LaPierre is on record in support of such a measure.

Sen. Boxer's school safety bill is even more popular, for obvious reasons. It would increase funding to expand security measures on school campuses.

Neither measure has faced any significant opposition.

Won't pass
There's a reason the assault weapons ban is being voted on as an amendment to the main package: Reid doesn't want its unpopularity to poison support for the rest of the measures. Instead, it will pass or fail on its own — by which we mean it will fail on its own.

The bill is unpopular enough that the NRA is using it as a smear against gun proposals in general. The reasons for its unpopularity are myriad but center around how squarely it fits with the NRA's messaging that the government wants to take guns away from legal owners. The NRA beat Feinstein on the issue in 2004 when the original ban was up for renewal; it's done so again.

Might pass
Bringing us to background checks. The key issue of contention here is record-keeping. A majority of both Democrats and Republicans agree that expanding background checks to any gun sale makes sense. The public agrees.

But negotiations on a bipartisan proposal broke down between Senators Chuck Schumer and Tom Coburn over whether or not sellers would need to keep a record of the checks they run. For some advocates, such a move would facilitate a national database of gun owners (though such a thing is prevented by federal law). When Schumer and Coburn couldn't cut a deal, Schumer moved his own initiative including only the aspects of legislation Democrats wanted to see.

When Reid was first putting together the package for the Senate to vote on, he considered having background checks be an amendment as well. But Reid appears confident that a bipartisan deal can still be worked out — optimism that may be rewarded, according to a report in The Hill. According to the paper, Senator John McCain has held conversations with other senators in an effort to reach a deal, though he's not publicizing the content of those conversations. The Hill notes some compromises Democrats have already embraced.

The proposal includes modifications to attract Republican support. One would let rural gun owners conduct background checks from their home computers. Another would give military veterans who have been declared mentally unfit to own a gun a process for appealing that finding.

Neither of these sweeteners resolves the core debate over record-keeping, which at least one senator, Nevada's Dean Heller, indicates is a blocking objection.

Wild card
New York City Mayor Michael Bloomberg hopes to change Heller's mind. Over the weekend, Bloomberg announced that Mayors Against Illegal Guns, an advocacy organization he founded and leads, would spend $12 million on ads in thirteen states with wavering senators.

Here's the ad.

The group's goal with the spot is to simplify how the public understands the debate, to make the choice in the public's eye between keeping guns out of the hands of those who shouldn't have them, or not. But we already know that the public's view isn't necessarily what motivates the Senate. Bloomberg's public and publicized action may help — but McCain's private ones may be more likely to get a deal done.