Blue Dog Democrats continue to see some concerning poll numbers.

The latest is Rep. Stephanie Herseth Sandlin (D-S.D.), who, according to a new Public Policy Polling (PPP) survey, could have a tough reelection race on her hands in 2010.

This cycle Herseth Sandlin has drawn her first serious challengers since 2004, and PPP shows her leading one of them, Secretary of State Chris Nelson (R), by a relatively modest seven points, 46-39.

Herseth Sandlin still has solid approval ratings, at 49 percent positive and 38 percent negative, but her seven-point lead over Nelson is concerning both because she is under 50 percent and because 59 percent of South Dakota voters don't know Nelson.

Herseth Sandlin joins members — including Reps. Frank Kratovil (D-Md.), Vic Snyder (D-Ark.) and Dina Titus (D-Nev.) — in seeing disconcerting early poll numbers. In addition, it has been revealed that Rep. Bart Gordon (D-Tenn.), who announced his retirement Monday, trailed in some early polling.

If other Blue Dogs and members in swing districts are seeing the same thing, that could lead to more retirements.