This house is representing somebody (Joshua Roberts/Reuters) (Joshua Roberts/Reuters)

This sentence scarcely seemed possible less than a year ago: On Thursday 68 senators voted to proceed to debate on major gun-control legislation.

Hold (some of) your applause. The bill is just good — not great — and it’s unlikely to get any better.

The base legislation would mandate background checks on nearly all gun purchases, put money into school security and crack down on gun trafficking. The deal Sen. Joe Manchin (D-W.V.) just struck with Sen. Pat Toomey (R-Penn.) to make the bill more GOP-friendly only slightly weakens the background check requirement. That’s good. The current federal background check system is a loophole-ridden farce, allowing at least 40 percent of private gun sales, including huge volumes online and at gun shows, to go on without federal review. Lawmakers should consider fixing the system to be required legislative maintenance, not some contentious new policy.

Instead, it seems to be the only big thing the Senate can manage to pass, and only if gun-control advocates win the battle over amendments to the bill.

On that, Senate procedure might well prove a double-edged sword for gun-control advocates. Democratic leaders were very cautious in what they included in their base bill, and a senior Senate aide told me that it’s likely any changes will require 60 votes to pass.

The Post’s Chris Cillizza and Paul Kane warned on Wednesday that senators might approve NRA-backed amendments on a simple-majority basis, gutting what’s already in the bill. But it looks like anti-gun control lawmakers will need a supermajority to dismantle the legislation. A 60-vote bar on amendments would also help ensure that the whole package that emerges from this process can attract another 60 votes to proceed to final passage, though that’s far from definite.

Yet that threshold also means reformers would also have a hard time making the bill stronger. The legislation contains no ban on military-style assault weapons, firearms no one needs for hunting or protection. It doesn’t even propose a prohibition on large ammunition magazines that have allowed mass shooters to fire dozens of rounds before stopping to reload. Senate Majority Leader Harry M. Reid (D-Nev.) has promised separate votes on adding these policies, but it’s hard to see them attracting 60 votes or more. Several of the senators who voted to proceed to debate on Thursday have expressed skepticism, or outright opposition, even to clip-size limits. So, gun-control backers have a shot at getting what they already have through the amendment process. But they probably won’t get much more.

It’s infuriating that a sensible, popular policy such as limiting magazine sizes — an inconsequential abridgment of 2nd Amendment rights, if it’s one at all — probably won’t get a simple up-or-down vote, in what is likely to be the last good opportunity for federal gun-law reform in years. Even so, reformers should take the win they can get on this legislation — fixing background checks is too attractive to do otherwise. Then they should channel their frustration into pressing for more policy later. The gun control debate cannot be allowed to end after this bill.

Update, 9:26 a.m.: Several commenters have disputed the claim that 40 percent of private gun sales occur without a federal background check. Fine. The fact remains that there are obvious loopholes in the background check system. Whether the number is 15 percent, 30 percent or 40 percent, the argument that these loopholes should be closed stands.

Also, Wonkblog points out that expanded background checks AND penalties for gun trafficking make the Senate guns bill worthwhile. I should have made that point clearer.

Stephen Stromberg is a Post editorial writer. He specializes in domestic policy, including energy, the environment, legal affairs and public health.