What is Driving Democrats' Votes on the Iran Nuclear Deal?

The US Congress is set to consider a measure to block the recent Iran nuclear agreement (brokered by the US, Great Britain, France, Germany, China, and Russia), when it return to session next week. To pass, the measure needs 60 votes in the Senate and, in the face of a likely presidential veto, a two-thirds majority. By seeming universal consensus, no Republicans are expected to oppose the measure to block the deal. Veto-proof support for the nuclear deal will require 34 votes in Congress--therefore, the President has been focusing on securing 34 votes among the Democrats.

Two Democratic Senators have publicly announced that they will vote in favor of blocking the deal: New York's Chuck Schumer and New Jersey's Bob Menendez.  Those in favor of the deal have been tenaciously announcing support, including most recently Senator Bob Casey of Pennsylvania (see recent NYTimes coverage on these announcements).

If much of the divide in Congress over the Iranian nuclear deal can be explained by partisan politics, nonetheless, Members of Congress and Democrats in particular in this case are likely feeling the pinch between remaining responsive to state and local constituents and also fulfilling their sense of national and foreign policy responsibilities.

These twin tasks may be made more complicated in light of the American public's seeming ignorance and ambivalence on the issue. According to PollingReport.com, a recent CNN poll found that 41% of Americans say that Congress should support the deal while 56% say Congress should reject the deal.  If this seems to suggest Americans are thoroughly divided on the issue (much like the US Congress itself), another poll conducted by Monmouth University suggests that 38% of Americans say they have heard only "a little" about the Iranian deal to begin with. Additionally, in this survey, only 32% of Americans indicated that Congress should approve the deal.  Public opinion is likely not a useful guide for lawmakers weighing pros and cons. For Casey, the picture is even more complicated, since 61% of PA voters say they oppose the deal with Iran and only 26% support it (read more: http://www.politico.com/story/2015/08/poll-iran-deal-oppose-florida-ohio-pennsylvania-121655#ixzz3kXIJuWZm).

To muddy the picture further, conventional wisdom suggests that the Presidency is the leading force in foreign affairs (and it's difficult to argue that Obama has not been getting his way in this policy domain) and that Congress often trails along.

Putting this all together, I'm interesting in thinking about the obligations and responsibilities of Members of Congress in this context. How ought lawmakers weigh citizens' expressed interests (recognizing how uninformed and capricious these interests might be) alongside Congress's interests (as an institution in need of asserting itself, quite frankly) alongside lawmakers' responsibility to consider national interests?