CNN

(CNN) – Pennsylvania Sen. Arlen Specter didn't only shock the political world with his decision to leave the Republican Party Tuesday, he made history.

Specter is the first senator elected as a Republican to switch directly to the Democratic Party.

Overall, Specter is the 13th senator since the direct election of U.S. Senators was established in 1913 by the 17th Amendment to have switched parties while in office.

And in what good be a good sign for Specter, every party-switcher who ran for re-election on his new party's ticket retained his Senate seat.

More info after the jump

Of the 12 who switched parties before Specter, none have ever switched directly from the Republican Party to the Democratic Party. Six started out as Republicans:

One switched from Republican to Independent, then two years later switched to Democrat (Wayne Morse of Oregon)
One switched from Republican to Independent but caucused with the Democrats (James Jeffords of Vermont)
One switched from Republican to Independent and stayed an Independent (George Norris of Nebraska)
One switched from Republican to Independent, then back to Republican five months later (Bob Smith of New Hampshire)
Two switched from Republican to Progressive

Six party-switchers started out in the Democratic party or the local equivalent:

Three switched from Democrat to Republican
Two switched from Democrat to "Independent Democrat" (Joe Lieberman of Connecticut and Harry Byrd of Virginia)
One switched from the Minnesota Farmer-Labor party (a forerunner of the modern Democratic party in that state) to Republican

RE-ELECTION PROSPECTS

Voters have reacted favorably when previous party-switchers ran for re-election. Every party-switcher who ran for re-election on his new party's ticket retained his Senate seat. Only one party-switcher lost a re-election bid; that Senator (Bob Smith of New Hampshire) lost when he returned to his original party and was defeated in the next primary for his seat. Of the 12 who switched parties before Specter:

Nine won re-election as a member of their new party
One switched back to his original party and was defeated in primary when he ran for re-election
Two did not run for re-election (one retired; one died in office)

List of the sitting U.S. Senators who switched parties after they were popularly elected:

Robert LaFollette, Sr.
Wisconsin
1924
Republican to Progressive
Died in office; never ran for re-election

Robert LaFollette, Jr.
Wisconsin
1934
Republican to Progressive
Re-elected as a Progressive in 1934

George Norris
Nebraska
1936
Republican to Independent
Re-elected as an Independent in 1936

Henrik Shipstead
Minnesota
1939
Farmer-Labor to Republican
Re-elected as a Republican in 1940

Wayne Morse
Oregon
1953,1955
Republican to Independent (1953);
Independent to Democrat (1955)
Re-elected as a Democrat in 1956

Strom Thurmond
South Carolina
1964
Democrat to Republican
Re-elected as a Republican in 1966

Harry Byrd
Virginia
1970
Democrat to "Independent Democrat"
Re-elected as an Independent in 1970

Richard Shelby
Alabama
1994
Democrat to Republican
Re-elected as a Republican in 1998

Ben Nighthorse Campbell
Colorado
1995
Democrat to Republican
Re-elected as a Republican in 1998

Robert Smith
New Hampshire
July to November, 1999
Republican to Independent
(July, 1999);
Independent to Republican
(November, 1999)
Lost Republican primary for U.S. Senate in 2002

James Jeffords
Vermont
2001
Republican to Independent
Did not run for re-election

Joe Lieberman
Connecticut
2006
Democrat to "Independent Democrat"
Lost Democratic primary for U.S. Senate in 2006; left Democratic party and won re-election in November 2006 as Independent Democrat

Arlen Specter
Pennsylvania
2009
Republican to Democrat
To come