Philosoraptor

Because the federal Senate is already a senior position in the party to begin with, and switching sides means losing your seniority in the party. If you include state senators as well as representatives and governors, the list becomes much much larger.

1960s – Arthur Ravenel, Jr., South Carolina State Representative, later U.S. Representative from South Carolina (1987–1995)

1960s – James F. Byrnes, former Governor of South Carolina (1951–1955) and United States Secretary of State (1945–1947)

1960 – Claude R. Kirk, Jr., later Governor of Florida (1967–1971)

1960 – Charlton Lyons

1961 – Perle Mesta, former United States Ambassador to Luxembourg (1949–1953)

1962 – Dave Treen, later U.S. Representative from Louisiana (1973–1980) and Governor of Louisiana (1980–1984)

1962 – Jack M. Cox, former Texas State Representative

1962 – James D. Martin, later U.S. Representative from Alabama (1965–1967)

1962 – Ronald Reagan, while an actor and former Screen Actors Guild president. Later 40th President of the United States (1981–1989)

1962 – Floyd Spence, South Carolina State Representative, later a U.S. Representative from South Carolina (1971–2001)

1963 – Rubel Phillips, former Mississippi Public Service Commissioner

1963 – Stanford Morse, Mississippi State Senator

1963 – James H. Boyce

1964 – Alfred Goldthwaite, Alabama State Representative

1964 – Clarke Reed

1964 – Howard Callaway, later U.S. Representative from Georgia (1965–1967) and United States Secretary of the Army (1973–1975)

1964 – Iris Faircloth Blitch, former Georgia U.S. Representative (1955–1963)

1964 – Charles W. Pickering, later Mississippi State senator and Judge of the United States District Court for the Southern District of Mississippi (2004)

1964 – Strom Thurmond, while U.S. senator from South Carolina (1954–2003).

1965 – Albert W. Watson, while U.S. Representative from South Carolina (1963–1971) (resigned before switching parties and regained his seat in a special election)

1965 – Roderick Miller, Louisiana State Representative

1966 – Marshall Parker, South Carolina State Senator

1966 – Joseph O. Rogers, Jr., South Carolina State Representative

1966 – Len E. Blaylock, later U.S. Marshal for the Eastern District of Arkansas (1975–1978)

1966 – Jerry Thomasson, Arkansas State Representative

1966 – Henry Grover, Texas State Representative

I am out of space in the box.

Indeed. There were also some dynasties switching sides from one generation to the next that don't qualify for the list but the effect is the same when the voters weighed in.