ARG's latest daily tracking poll has Kerry 31%, Clark 20%, Dean 18%. That means that from January 19th to January 22nd Dean fell from 28% to 18%. In fact, from yesterday to today he fell 4 percentage points.

Remember, a tracking poll like this combines three days of calls together. So this evenings numbers are the first which don't include any calls from before caucus night in Iowa. Pulling that last batch of the pre-Iowa numbers out of mix probably accounts for that rapid four point fall.

Meanwhile, the other tracking poll out tonight from the Boston Globe and WBZ-TV has Kerry 34%, Dean 19%, Clark 14%, Edwards 11%. The big difference there is a much sharper deterioration for Clark.

Late Update: Friday morning's Zogby numbers show the same story for Kerry as ARG, but a less acute fall for Dean, a continued slide for Clark, and no movement for Edwards.