Even the hint of an organized anti-Sanders movement would risk alienating the Vermont senator's sometimes belligerent supporters and play into claims that the process is "rigged," many Democrats say privately. Democratic House candidates in swing districts say they are nervous about running on the same ticket as Sanders, but they, too, are reluctant to say so publicly.

That's leading some Democratic centrists to warn that the silence carries a risk of waiting until it's too late.

"People need to start taking Bernie pretty seriously - there is a really substantial risk of him becoming unstoppable if he wins these early states by large numbers," said Matt Bennett, vice president of Third Way, a centrist Democratic group.

But despite "some discussions among people wringing their hands," Bennett said, "It's not like our phone is ringing from people saying, 'Let's do something.' "