Debates, Not Voters, Begin Sending Democratic Presidential Candidates Packing

The current tally of 20 Democratic presidential hopefuls is enough to set a record in any previous primary season. But even with the giant number of candidates, the reality is that the winnowing has already begun.

The field is shrinking — slowly — but what's different this time compared to past campaigns, is what's driving candidates to pack it in.

Here's what it's not — voters.

"Iowa really isn't the first winnowing process in the primary anymore," said Brad Anderson, a veteran Iowa campaign operative.

It's now the debates, he...