(J. Scott Applewhite/AP)

Four gun control proposals failed in Senate votes Monday — not a good sign for those who want to tighten our nation’s gun laws after Orlando.

But there may be one last chance for some sort of deal to emerge. It comes from one of the Senate’s most moderate Republicans, Susan Collins of Maine, who will officially unveil her proposal Tuesday afternoon. We’re unclear how much support it will have, but at the very least, it looks like both sides in a very divided Senate are willing to hear her out.

Here’s what we know about the Collins compromise — and why she’s proposing it.

What’s in it?

Right now, anyone on the FBI’s various terrorist watch lists can legally buy a gun. Senate Republicans and Democrats both agree that suspected terrorists shouldn’t be able to do that, but they’re split on whether to let the attorney general ban people from buying guns before or after the courts have a chance to weigh in.

Instead of preventing people on the various FBI’s terrorist watch lists from buying a gun as Democrats want, Collins’s proposal would allow the attorney general to prevent people on two specific lists from buying a weapon. (It’s unclear exactly many lists there are — for obvious reasons.)

Both lists deal with a person’s rights at the airport. If you’re on the no-fly list, you can’t board an airplane. If you’re on the selectee list, you get extra security screening when you try to board a plane. (Orlando killer Omar Mateen was on the selectee list for a time.) And if you fit the criteria for being on those lists — but for some reason the FBI never put you on it — the attorney general could also ban you from buying a gun.

Under Collins’s proposal, the attorney general doesn’t have to go to court first to prove you’re a suspected terrorist (as Republicans want). But if you are on these lists and are denied your right to buy a gun, you can challenge it afterward. And if you win, the government has to pay your legal costs.

Why these lists? Sen. Lindsey Graham (R-S.C.) said it’s because they’re the most serious of the FBI’s watch lists. “There are no Bubbas on this list,” he told reporters Monday. “If you’re on this list, it’s not because you went to a tea party rally or you’ve got a political ax for the president or you’re a liberal. You’re on this list because you’re doing things that unnerve the FBI to the point that you can’t fly on an airplane.”

One more thing: Collins’s proposal also would require the FBI to notify law enforcement if someone who had been on the terrorist watch lists in the past five years buys a gun. That doesn’t mean they couldn’t buy a gun, but at least law enforcement would know when someone who was once considered suspect did.

Why is this a compromise?

Collins’s proposal isn’t exactly what Democrats want (they don’t like the idea of working with just two terrorist watch lists) or what Republicans want (they don’t like the idea of barring a person from buying a gun first, then offering legal recourse later).

“Her alternative is not enough to close the loophole that creates this terror gap,” said Sen. Dianne Feinstein (D-Calif.) on Monday, according to The Washington Post’s Karoun Demirjian.

What are its chances?

Unclear. As with any real attempt at compromise, neither side seems thrilled with the outcome. But neither side is a hard no either.

Senate Minority Leader Harry Reid said Monday that he’s waiting to see if Collins’s proposal could “drum up the 20 votes or so” from Republicans to get the estimated 60 votes it needs to actually pass. (A Republican version of the terrorist watch list proposal failed 53 to 47 on Monday, and a Democratic version failed 47 to 53.)

Senate Majority Leader Mitch McConnell said he was withholding judgment until the bill was actually released.

So far, leading gun control groups and the National Rifle Association have been silent on it, too.

It seems like the only ones talking about Collins’s proposal are moderate lawmakers in both parties:

Huge caveat here: We’re only talking about the proposal’s prognosis in one chamber. Over in the more conservative House of Representatives, Speaker Paul Ryan has indicated he may not support restricting gun purchases to people on any terror watch list.

Unless Ryan’s position changes, it’s very hard to see how this proposal — even if it passes the Senate — becomes law.

What does this all mean in the context of gun control?

That passing any new gun control laws is still really, really difficult. And that any potential compromise will have to be narrow enough to squeak through without upsetting the powers that be on either side of the debate.