Yet another poll shows that very strong majorities of Americans want to let the Bush tax cuts that only benefit Americans earning more than $250,000 a year expire. Normally when an issue like this has such lopsided standing with the public, you'll see moderates from the party standing on the unpopular side defect to the popular side. Instead you're seeing just the opposite:

On Thursday, Nelson said in a statement that he wants to extend the cuts for all taxpayers, but did not specify a time frame for ending the cuts, saying only that they should last "until Nebraska's and the nation's economy is in better shape, and perhaps longer, because raising taxes in a weak economy could impair recovery."

Lieberman used the same reasoning in a recent interview with a Connecticut radio station. "We've got to be really cautious, because we don't want to do anything that would put the economy into a second dip and cause more loss of jobs," he said.

Conrad, who chairs the Senate Budget Committee and voted against the Bush tax cuts in 2001, has said that an extension of all tax cuts is necessary, at least for a while. "As a general rule, you don't want to be cutting spending or raising taxes in the midst of a downturn," he told the Wall Street Journal.

Why is this happening? One possibility is that vulnerable red state Democrats are trying to signal their moderation by breaking with the party leadership on a high-profile issue, even one in which the party has a popular stance. But the defections include retiring Democrats like Bayh and Conrad, and a blue state Senator like Lieberman whose main political danger comes from the left.

So my explanation is that Senators are both among, and surrounded by, the small minority of Americans who earn more than $250,000 a year. They hear from them disproportionately, they live among them, and they are them. So their conception of what is popular and what is reasonable on this issue is warped.