On Tuesday, the Senate will have once last chance to try to block the Iran nuclear deal. Last week, President Obama won a filibuster with the help of 42 Senate Democrats to block the resolution to disapprove it. Senate Republicans will bring the resolution up for another vote Tuesday

If one of those 42 senators is yours, get on the phone right now to get him or her to switch that vote.

You can tell your senator that verification of the deal will be even harder than anyone realizes--thanks in part to Edward Snowden.

I say this as a former Navy intelligence officer who was on active duty during the 1979 Iranian revolution.  We learned firsthand the deep hatred the revolutionary mullahs had for the US after its long support for the Shah.  That hatred has not abated in 36 years.  It’s fueled Iran’s military objective to get a nuclear weapon to cement its dominance in the Islamic world.

So how big is the challenge of monitoring the deal and what are the potential consequences?

The three primary pillars of intelligence collection are imagery, technical monitoring and human intelligence.  The US will need to leverage various components of each to even have a chance of verifying whether Iran is complying with the agreement and to assure it’s not surprised someday by an Iranian nuclear weapon test, as it was with North Korea and Pakistan.

Imagery:  From space-based systems, drones, aircraft or sources on the ground, imagery collection is pretty straightforward.  Most sophisticated countries are aware of the capabilities and limitations of imagery, and countries practice well-known methods to confound, confuse or deny intelligence from imagery collection.  That’s why North Korea (from which Iran has obtained assistance with its nuclear program) builds elaborate, deep underground facilities.  This not only makes attacking them a challenge, but expansion and configuration changes can be carried out with minimal exposure to “prying eyes.”  Some of Iran’s key nuclear facilities are underground.

Technical monitoring:  This involves either electronic transmissions or information technology sources.  This is the area you hear about in the news these days regarding computer hacking.  In this discipline, many analysts have failed to consider the “Snowden effect” with Iran.

While Snowden’s stated intent of his intelligence theft was to alert his fellow Americans to threats of “Big Brother,” he also aided and abetted our enemies with these disclosures. While I’m confident the US intelligence community has done a thorough damage assessment post-Snowden and is well on its way to repairing its collection capabilities or developing new methods to assess information, Snowden’s releases have given hostile countries insights into exploitable trends and systems the US uses in intelligence collection.

But also consider this: Snowden is currently living in asylum in Russia, and Russia wants desperately to expand its political and economic relationship with Iran.  We should assume that Russia (and possibly China, as Snowden also spent time in Hong Kong) has exploited much of Snowden’s pilfered data and knows very well what our targets, methods and success rates are.  If Russia shares with Iran even its general insights on Snowden’s disclosures of our intelligence targets and capabilities, Moscow could strengthen its relationship with Tehran immeasurably.

Human Intelligence:  Without direct access by US personnel, it will be difficult for the US to get valuable first-hand information to stop Iran’s nuclear ambitions.  The IAEA is comprised of inspectors from many countries, some friendly to the US and some not.  These inspectors report to the UN, not the US, and while they may have direct access to some of Iran’s nuclear sites and data, their reports will be at best second hand and also won’t necessarily be focused on US intelligence needs for verification. At worst, they’ll be unreliable--remember the UN WMD inspectors during the Iraq War?

There’s nothing like access from a source you can control, and we don’t have it in this deal. Let’s pray our clandestine efforts in Iran are successful. We’ll need them.

And after all of this, the fundamental issue may not be these challenges in intelligence and verification. For our allies, it’s our propensity of late, with our demonstrated short attention span, to take our eye off the ball when we become absorbed in the next crisis. How many times in the last few years have we lost focus on a serious national security issue? Without solid, coordinated intelligence, the prospect of Israel or Saudi Arabia going it alone to strike Iran preemptively becomes a frightening possibility. And that could lead to an unmitigated disaster of a nuclear exchange in the Middle East, in which we would inevitably become involved.

Our leverage?  As critics of the deal have noted repeatedly, the financial sanctions imposed on Iran are the only leverage we have with Iran short of direct military action.  Once they are removed, Iran will have the resources and flexibility to pursue its agenda of regional hegemony.  Instead, the sanctions should stay in place until the deal is renegotiated to include stronger verification provisions.

We can get a better deal through the pressure of continued sanctions.

But it’s clear these operational principles are not the primary factors driving decisions on this agreement. It’s politics and the president’s legacy objectives--not the cold, hard facts.