Castle will make a run for Senate.

(CNN) - Rep. Mike Castle, R-Delaware, will announce at noon Tuesday that he will seek the Senate seat once held by Vice President Joe Biden, two sources tell CNN.

Castle is a former governor who was elected to the House in 1992. His entry into the race triggered the non-partisan Rothenberg Political Report to characterize the contest late Tuesday morning as a "Lean Takeover" in its 2010 race ratings.

Democrats are now waiting to see if Vice President Biden's son, Beau, declares his candidacy for the seat. Beau Biden is the Delaware attorney general. He recently returned home after spending a year with his Army National Guard unit in Iraq.

– CNN's Paul Steinhauser contributed to this report