As I keep telling you, the prospects for immigration reform may turn on a key question: Are Republicans prepared to accept a path to citizenship that is not pre-conditioned on the meeting of hard border security metrics as “triggers” for citizenship to proceed? There is no papering over this question. The answer is either Yes, or No.

But don’t take my word for it. In a new interview with the Washington Examiner, Marco Rubio — who is key, since many Republicans are taking their cues from him — tacitly admits this to be the case. Rubio’s words need to be parsed closely, because they perfectly capture the challenges that lie ahead.

Rubio is set to introduce an amendment to the gang of eight bill that, he hopes, will strengthen its border security provisions in ways that will win over Republicans. But, notably, he carefully places some distance between himself and John Cornyn, who has insisted that citizenship must be contingent on hard security triggers. Rubio:

“It’s critically important that we detail in the bill what the border plan and the fence plan is, because there’s just no trust…So, what I’m arguing is that we should make that plan detailed. Let’s detail what the border plan is, so when members vote for this bill, they’re not voting for the promise of coming up with a border plan in the future, they’re voting on a border plan that we have seen, that we have talked to border patrol agents about and that we actually know how many miles of fence, how many sensors, how many cameras, what we’re actually asking them to complete before the green card process can start.

“The second aspect of it that we’re going to have to have a debate about is: How can we measure? So, not just input based. But how do we measure on the back end how the program is working? How do we measure whether it’s successfully been implemented?

“[The Democrats] don’t want anything to make the path to citizenship uncertain. But it already is conditioned. The path to citizenship in this bill — the path to that green card — it’s already conditioned on the full implementation of E-Verify. It’s already conditioned on the full implementation of the entry-exit tracking system. And, the last thing is, it’s got to be fully conditioned on the completion of the specific border plan that we detail.”

This is critical. Rubio’s amendment will focus on developing a specific plan for border security up front, rather than leaving that in the hands of the Department of Homeland Security, as the gang of eight proposal stipulates. Rubio knows that this is acceptable to Democrats, while setting hard triggers at the back end isn’t. In this sense, Rubio is carving out a space that’s distinct from that inhabited by Cornyn. At the same time, Rubio also knows that hard triggers are a powerful lure for Republicans, as evidenced by the fact that they are gravitating towards Cornyn’s plan. That’s why Rubio is careful to allude to the need for a way to “measure on the back end how the program is working.”  With these remarks, Rubio has put his finger on the key distinction that may decide whether reform happens.

Rubio hints at his solution to bridging this divide with his suggestion that citizenship must be “fully conditioned on the completion of the specific border plan that we detail.” Again, the emphasis here is on the front end plan, rather than on hard metrics for judging success later. Yet Rubio is also hinting that such metrics will, in some form or other, be part of this plan.

Ultimately what all of this comes down to is that Republicans don’t trust Democrats to make good on border security, while Democrats don’t trust Republicans to make good on citizenship. But as Rubio admits above, the current compromise already contains triggers as preconditions for citizenship, in the form of E-Verify and the entry/exit tracking system. Dems have also agreed on billions of dollars for border security, and they are also willing to put the security plan in the hands of Congress, rather than in the hands of DHS, as Rubio wants.

Perhaps he can come up with some kind of metric that is acceptable to both sides. But he very well may not be able to, because Dems may well hold fast to their position that any hard trigger metrics that are subject to manipulation deliberately designed to scuttle citizenship are a nonstarter. In which case Republicans will need to decide whether everything Dems are already giving them on security is enough. Rubio’s position is not an enviable one. He’s got a very tough needle to thread.

* OBAMA APPROVAL PLUMMETS IN NEW POLL: A new CNN poll finds that Obama’s approval has dropped eight points in the last month, to 45 percent, with the percentage who think he is “honest and trustworthy” below 50 percent for the first time in his presidency. The poll also finds 61 percent disapprove of Obama’s surveillance. Yet at the same time, majorities support the NSA programs, with 51 percent approving of the phone records gathering and 66 percent approving of the online data gathering.

The eight point swing seems particularly dramatic — the Pollster.com polling average shows a drop from last month, but only a slight one.

* YOUNG VOTERS TURNING ON OBAMA? Another key finding in the CNN poll:

“The drop in Obama’s support is fueled by a dramatic 17-point decline over the past month among people under 30, who, along with black Americans, had been the most loyal part of the Obama coalition,” says CNN Polling Director Keating Holland.

However, the poll shows pretty much equivalent disapproval of the surveillance programs among all age groups, so it remains unclear whether the NSA revelations account for this.

* IS THE GOP IN A “DEMOGRAPHIC DEATH SPIRAL”? So says Lindsey Graham. Yeseterday on Meet the Press he said that if Republicans don’t help pass immigration reform, they can’t win the presidency in 2016:

“We’re in a demographic death spiral as a party and the only way we can get back in good graces with the Hispanic community in my view is pass comprehensive immigration reform. If you don’t do that, it really doesn’t matter who we run in my view.”

Immigration reform’s prospects turn on whether enough Republican decide the same thing — if enough of them privately want it to pass, whether or not they are prepared to vote for it, it will pass.

* REPORT: OBAMA CONSIDERING DECLASSIFYING NSA OPINION: NPR reports that the Obama administration is mulling whether to declassify a Foreign Intelligence Surveillance Court opinion authorizing the NSA to seize the phone records of millions of Americans. If true, this is a welcome first step. As noted here on Friday, it appears the administration could declassify these opinions if it wanted to (Congress could compel it, too), which would make it possible for us to have that “debate” over the proper liberty/security balance Obama says he wants.

* ANOTHER IRS OFFICIAL GIVES CONFLICTING TESTIMONY: USA Today reports that a high ranking IRS official testified to House investigators that the term “Tea Party” internally referred to any political group, not just conservative ones — more confirmation liberal groups were also targeted. However true or relevant this proves, the key is that this testimony emerged because USA Today gained access to extensive committee transcripts that have not been publicly released.

This comes as Dems are increasing pressure on GOP Rep. Darrell Issa to release committee transcripts. Issa is refusing, laughably claiming the release of full transcripts, as opposed to selective snippets, will prejudice the investigation.

* MARKEY HOLDS COMFORTABLE LEAD IN MASSACHUSETTS: A Boston Globe poll released over the weekend shows Ed Markey leading Gabriel Gomez by 13 points, 54-41, with only four percent undecided. The Real Clear Politics average of polls puts Markey ahead by 9.6 points — a very big hurdle to overcome with only one debate and eight days remaining.

* AND FORGET ABOUT THAT “GRAND BARGAIN,” DEMS: Paul Krugman nails the coffin shut on any grand bargain on entitlements, explaining why it’s utter folly to keep chasing it:

Republicans in Congress have voted 37 times to repeal health care reform, President Obama’s signature policy achievement. Do you really expect those same Republicans to reach a deal with the president over the nation’s fiscal future, which is closely linked to the future of federal health programs? Even if such a deal were somehow reached, do you really believe that the G.O.P. would honor that deal if and when it regained the White House?

When will we be ready for a long-run fiscal deal? My answer is, once voters have spoken decisively in favor of one or the other of the rival visions driving our current political polarization.

In other words, with the possible exception of immigration, we may be stuck in stalemate until at least 2016, and likely beyond.

Happy Monday!

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.