Secretary of State John Kerry sits with Iranian Foreign Minister Javad Zarif in New York in September. (Stan Honda/AFP/Getty Images)

I'm about to attempt something you rarely see done by foreign policy folks from any point of the political-ideological spectrum: express sympathy for Congress and its role in U.S. foreign policy. I know, I'm surprised too, but bear with me. It has to do with its dilemma on Iran. Or, rather, its four dilemmas.

The Obama administration is working toward a possible deal with Iran and the European Union over Iran's nuclear program. It would most likely work this way: The Americans and Europeans give Iran some relief from economic sanctions, and in return Iran halts or rolls back its nuclear activities.

Here's where Congress comes in: If the United States does strike some kind of deal, eventually Congress will have to approve a reduction, whether permanent or temporary, in Iran sanctions. In the meantime, the White House has asked lawmakers not to pass any new sanctions, which would undercut U.S. diplomacy and risk sending the message to Tehran that they can't trust the Americans to deliver on their end of any bargain, either because Washington isn't negotiating in good faith or because Obama is powerless to deliver. No one is sure how Iran would react, but given that even talking to the United States is deeply controversial in Tehran, there's good reason to worry that Iranians would walk away.

Congress, being Congress, has ignored this and is pushing ahead with new sanctions anyway. The Obama administration is so worried that it dispatched Secretary of State John Kerry to try to talk them out of it.

Members of Congress who support imposing new sanctions right now have offered a less-than-persuasive case: first, that Congress should punish Iran for failing to unilaterally roll back its nuclear program, which is strange as it's not clear why Tehran would want to weaken its own position going into negotiations, and; second, that if sanctions brought Iran to the negotiating table, then more sanctions will keep them there. That last point would seem to contradict basic principles of negotiation, in which escalation normally pauses during talks, to demonstrate good faith and avoid pushing away your partner.

Okay, you might be saying, so far this post is not expressing much sympathy for Congress. But U.S. lawmakers are facing four dilemmas on Iran sanctions. On their own, these are all pretty minor, but taken together you might start to get a sense why lawmakers are so hesitant to follow the White House on Iran sanctions.

First, and maybe most importantly, U.S. sanctions on Iran are about more than just the nuclear stuff that's currently on the negotiating table.

"Those sanctions, in almost every case, are predicated not just on Iran's nuclear activities but also on their support for international terrorism, their opposition to the Middle East peace process and a whole list of other things," Kenneth Pollack, an Iran scholar at the Brookings Institution, told me recently. Pollack warned, "I think it's going to be very hard for the president to go to Congress" and ask for sanctions relief just on the specific nuclear issue, as those issues are left unaddressed.

The dilemma here is that the only way for Iran to address all of these issues at once would be in a grand bargain. Given how hard it is just for Tehran and the Western powers to come together on the nuclear issue, a grand bargain probably isn't likely. At least not all at once; if Congress wants all of these Iranian issues addressed, it will probably need to go through them one at a time.

Second, as Tufts University professor and Foreign Policy magazine blogger Dan Drezner points out, members of Congress might be validly concerned about "the Obama administration's distinguished record of bollixing up its Middle East diplomacy." (I wrote earlier on that record, and how the administration's internal politics might explain it.) In other words, a member of Congress who sees the administration's vacillating approach to Syria or Egypt might worry that, even if Congress holds back on sanctions to help out the administration's diplomacy, the administration might not come through on its end and then sanctions will have been paused for nothing.

Third, also from Drezner, Iran has earned something of a reputation for "evading" the International Atomic Energy Agency (the United Nations nuclear watchdog) on inspections. So lawmakers are weighing their willingness to trust Tehran in their calculations. Of course, Congress doesn't need to actively trust Iran to hold off on passing new sanctions -- lawmakers can always just do it later -- but its members do at least need to temper their feelings of mistrust.

Fourth, and most simply, Iran is very unpopular in the United States. Republicans are already signaling that they may use the Obama administration's Iran outreach as a political weapon in coming elections. Any lawmaker who votes against new sanctions, even if it is for very sound foreign policy reasons, is taking a big political risk. Democrats risk being targeted as part of a broader political campaign in 2014 or 2016. Maybe so do Republicans, who are already worried about getting on the wrong side of a conservative wing that's been perfectly happy to unseat legislators who don't toe the line.

It's not hard to see how legislators could conclude that, if Iran becomes a big political issue in coming elections, it won't matter whether U.S. diplomacy works, or if holding back on new sanctions was the right call. Even if a deal does succeed, all the political credit would likely go to the Obama administration.

To be clear, in foreign policy terms, it still seems pretty obvious that new sanctions would be high-risk and offer little potential reward. Even if you believe that Tehran absolutely, positively cannot be trusted -- as some right-leaning foreign policy hands do -- then Congress can still pass new sanctions at any time. Still, lawmakers are facing a pretty daunting cost-benefit analysis when it comes to the sanctions question. They personally stand to gain little from complying with the White House's request and may be putting themselves at risk. It's not exactly profiles in courage, but perhaps this is just how things work.