Who cares if U.S. public opposes Iran deal?

National Journal's Josh Kraushaar goes deep into new and old polling data and finds some interesting historical comparisons for public opinion on the Iran deal: "Polling of [foreign] policy: 53% opposed Panama Canal treaty, half approved Marshall Plan, 72% supported war in Iraq. 21% back #irandeal." We have to be careful about the polling on Iran because we've seen different results based on the wording of the question. This probably indicates that most people don't have strong views on the subject.

Still, let's suppose the deal really is highly unpopular with the American public. What should everyone -- and, more important, politicians -- make of it? Practically nothing.

For almost all politicians' electoral purposes, what matters on foreign-policy issues, especially complex ones, is the outcome. The best guess for whether a position or vote is safe is whether the policy works in the long run, not whether it is popular before it is passed.

Look back at the controversy over the Panama Canal treaty in the mid-1970s. It was a huge issue, splitting the Republicans in particular. Once the treaty was ratified in 1978, however, it rapidly disappeared from U.S. politics. Ronald Reagan was the chief opponent of the agreement, but by 1980 it was such a dead issue that he didn't mention it in his convention acceptance speech, and the subject didn't come up in his debate with Jimmy Carter.

Of the foreign-policy battles Kraushaar mentions, the only one with big electoral consequences was the Iraq war. Its outcome badly hurt Republicans in 2006 and 2008 despite being popular in public-opinion surveys before the fact.

For a small group of politicians, there are instances when current opinion can be more important than what happens down the road. These are the lawmakers who have strong ties to organized constituency groups with strong views on the subject. Not only are those groups likely to remember if their representatives vote to "betray" them, they are also least likely to change their minds about the policy however it plays out in the future.

When it comes to the Iran agreement, however, most members of Congress should decide how to vote (if they're worried about how it will play out in future elections) based on what they believe will happen in Iran, not on how their constituents are reacting now to polling questions.

It turns out almost everyone in Congress is voting by party affiliation on this one. That may be because, given the uncertainty over how the deal will work out, they believe it's safer to stick with their party. Or it may be that both Republicans and (most) Democrats really believe different things about how it will all play out.

Jonathan Bernstein is a Bloomberg View columnist covering U.S. politics.