So what will conservatives demand, as their price for supporting immigration reform? Will they insist on tougher enforcement “triggers” that must be met in order to allow the path to citizenship to proceed? If so, that could kill reform, because Democrats may not be able to accept anything that undermines citizenship, which is central to making reform real and comprehensive.

Along these lines, pay very close attention to this quote from Senator Marco Rubio — one of the pro-reform Republicans on the “gang of eight” — in which he strikes a very careful balancing act. The quote comes in an interview with Univision:

“Let’s be clear. Nobody is talking about preventing the legalization. The legalization is going to happen. That means the following will happen: First comes the legalization. Then come the measures to secure the border. And then comes the process of permanent residence. What we’re talking about here is the system of permanent residence. As for the legalization, the enormous majority of my colleagues have accepted that it has to happen and that it has to begin at the same time we begin the measures for [the border]. It is not conditional. The legalization is not conditional.”

On the one hand, this is encouraging, because it means Rubio — whose continued support is pivotal to reform’s long term hopes — will not insist on any conditions for legalization. As Byron York puts it, this constitutes Rubio’s “strongest statement yet that legalization of the nation’s estimated 11 million illegal immigrants must happen before any new border security or internal enforcement measures are in place, and will in no way be conditional on any security requirements.”

At the same time, though, note that Rubio is also taking great care to say that border security must be achieved before “permanent residence” can happen. And here is the rub of the whole debate. It’s not enough for Rubio to say initial legalization comes first (that’s what the Senate gang of eight bill stipulates). We need to know whether Rubio is going to insist on hardened border security triggers as a condition for the path to citizenship — that is, we need to know whether he’ll insist that if certain metrics are not met, citizenship cannot happen.

Rubio is very carefully avoiding a direct answer to that question. The Senate “gang of eight” bill that Rubio supports does not contain any such border security metric trigger. It would require the Department of Homeland Security to create a new border security plan, allot billions to carry it out, and require the creation of a southwest border security commission if 90 percent of crossers aren’t being apprehended. But failure on those fronts would not sink the path to citizenship.

By contrast, GOP Senator John Cornyn is insisting that certain border metrics — 100 percent monitoring at every segment of the southern border; a 90 percent apprehension rate; a biometric exit system at every air and sea port of entry — must be met before any path to citizenship can be operative. Rubio has said the current gang of eight plan cannot pass the Senate, and he will push to make it more conservative, for instance, by requiring more specificity about how those billions will be spent and perhaps taking some of the spending decisions out of the hands of DHS. But we don’t know whether he’ll insist on a hardening of border security triggers as pre-conditions for the path to citizenship. He has said positive things about the Cornyn plan — after all, he needs to keep leaning right to signal to conservatives their interests are being protected in these talks. But we don’t know how far he’ll go in embracing it.

Democrats have said that anything that hardens those pre-conditions as a requisite for citizenship is a non-starter, because it will undermine the core of reform. That seems designed to call Rubio’s bluff. But it’s very possible Rubio simply doesn’t believe Dems are willing to sink reform over any GOP insistence on harder triggers. Which means he could still try for them, particularly if he worries that he’s overexposed to criticism from the right.

“Any attempt to harden those triggers would likely result in a threat to the path to citizenship,” Frank Sharry, the head of the pro-immigration America’s Voice, tells me. In a reference to the gang of eight compromise, Sharry adds: “Rubio has won the most massive expansion of enforcement in U.S. history. Isn’t that good enough? Is Rubio prepared to accept a path to citizenship without hardened triggers?”

That really is the core question, and we don’t know the answer to it. We will find out the answer in coming days, when we see the actual amendments Republicans push forward, and which ones Rubio and other pro-immigration gang of eight Republicans do support. Rubio’s careful language above alone suggests just how careful a line he’ll have to walk — and the degree to which any miscalculation on either side could still blow up reform.