The politics of the middle-class tax cut would seem to be crystal clear. Over the last few weeks, a half-dozen polls have shown that voters nationally want to extend tax cuts for the poor and middle-class but not the very rich, putting them squarely on the side of President Obama and Democratic leaders in their debate with the Republicans.

But not all Democrats in Congress are ready to follow their leaders. A handful recently sent House Speaker Nancy Pelosi a letter, calling for extension of all the tax cuts and not just those for the middle-class. And they're not the only ones who feel that way. During a meeting of the House Democratic caucus on Tuesday evening, Pelosi gave a passionate argument for voting on the middle-class tax cuts only. But, according to accounts in Talking Points Memo and other outlets, the caucus remains divided--enough that Pelosi isn't ready to force a vote yet.

Why the reticence, given the poll numbers suggest the leadership's position is so popular? It's possible these members have an honest substantive preference for extending the upper-income cuts, because they think it makes sense economically or philosophically, or because they hear a lot from that very small portion of the electorate that pay the upper income rates.

But the vote seems to be largely political, on the theory that what polls well in the country as a whole doesn't necessarily poll well in every district.

Greg Sargent, who's been following and writing about the polls at Plum Line, notes that "national polls don't matter as much as how this issue polls in the marginal districts across the country that will decide who controls the House of Representatives." And Democrats that represent more conservative district worry that their constituents are more likely to believe Republican spin on the tax cut.

Here's how a senior congressional aide, one in close touch with more conservative Democrats, explained the thinking:

Like it or not most independent voters (and even conservative Democrats) think the president is anti-business and fighting with republicans about their small business canard is counterproductive at this stage for most members in red states.

Of course, this aide also made two counter-arguments to that point of view: Obama hasn't been anti-business by any reasonable standard. And if a vote for extending only the middle-class tax cuts helps the party nationally, as it would seem likely to do, the party's stronger position overall could offset, or even overwhelm, the negative reaction of more conservative voters turned off by the vote.

But if you want to know why some Democrats are on the other side of the national polls, well, now you know the answer.