Michael "Mike" Newbold Castle (born July 2, 1939) is the U.S. Representative for Delaware's At-large congressional district, serving since 1993. He is a member of the Republican Party.
The district, which is the oldest in the nation, incorporates the entire state of Delaware. He is the longest-serving U.S. Representative in the state's history. Prior to his election to Congress, Castle served as a member of the Delaware General Assembly, first in the State House of Representatives (1966-1967) and then in the State Senate (1968-1976). He was the 20th Lieutenant Governor of Delaware from 1981 to 1985, and the 69th Governor of Delaware from 1985 to 1992.
On October 6, 2009, Castle announced his candidacy in the 2010 special election for the seat in the United States Senate currently held by Democrat Ted Kaufman. Kaufman was appointed by Governor Ruth Ann Minner to fill the vacancy created by Joe Biden, who resigned to become Vice President of the United States; and is not a candidate in the special election. The election will determine who will fill the balance of Biden's term, which ends on January 3, 2015. In one of the most surprising election results of the 2010 campaign season, Castle was defeated in the Republican primary for the US Senate seat by Christine O'Donnell. He would have been heavily favored against Chris Coons in the general election.