From NBC's Harry Enten
Arlen Specter made history today becoming the first-ever senator to switch directly from the Republican to the Democratic Party.
In the history of the republic, 20 other United States Senators have switched political affiliations while in office with 12 of those coming since the direct election of senators in 1913, according to records kept by the U.S. Senate Historical Office.
Most recently, Joe Lieberman (CT) switched from the Democratic Party to Independent Democrat. He caucused with the Democrats in 2006 after losing a primary to challenger Ned Lamont.
While Specter's and Lieberman's decisions were mostly due to political survival, this is not the norm for party switchers.
Some have done so out of a belief that they no longer fit ideologically. Strom Thurmond's (SC) switch from the Democratic to Republican Party in the watershed year of 1964 is an example.
Others have done so due to frustration with the party establishment. Harry Byrd (VA) switched from the Democratic Party to an Independent, who caucused with the Democrats in 1971. He refused to pledge an oath to supporting Democratic candidates, including the Democratic candidate for president in 1972.
More recent examples include Jim Jeffords (VT), Robert Smith (NH), Ben Nighthorse Campbell (CO) and Richard Shelby (AL).
In 2001, Jeffords left the Republican Party to caucus with the Democrats as an Independent. By doing so, he gave the Democrats a one-seat majority in the Senate. Although he had just won re-election, he, like Specter, came from a state that was becoming increasingly Democratic.
In 1999, Smith switched from Republican Party to Independent in order to seek the U.S. Taxpayers Party nomination for president in 2000. Five months later, he gave up running for president and switched back to the Republican Party.
After Republicans won back the Senate in 1994, Nighthorse Campbell and Shelby left the Democratic Party to become Republicans. Both gained seniority on important Senate committees.
[EDITOR'S NOTE: An earlier version of this post incorrectly identified Ben Nighthorse Campbell's state. It has since been corrected.]