Rep. Michael N. Castle, a Republican candidate for Senate from Delaware, here with his wife, Jane, addresses supporters after his defeat in the primary election on Tuesday, Sept. 14, 2010, in Wilmington, Del. Mr. Castle lost to "tea-party" favorite Christine O'Donnell. (AP Photo/Steve Ruark)
Rep. Mike Castle, left, a Republican candidate for U.S. Senate from Delaware, is helped off stage after addressing supporters after his defeat on primary election night Tuesday, Sept. 14, 2010, in Wilmington, Del. Christine O'Donnell defeated Castle. (AP Photo/Steve Ruark)
Republican Senate candidate Christine O'Donnell smiles while addressing supporters after winning the Republican nomination for Senate in Delaware, Tuesday, Sept. 14, 2010, in Dover, Ms. O'Donnell upset Rep. Mike Castle. (AP Photo/Rob Carr)
Rep. Michael N. Castle, who has held elective office in Delaware since the mid-1980s, finds himself unexpectedly behind "tea party" favorite Christine O'Donnell in the Republican primary for the Senate. (AP Photo)
Rep. Michael N. Castle, Delaware Republican, is taking his GOP rival for the nomination for Senate more seriously after a Republican senator was denied renomination by an insurgent in Alaska.