Pennsylvania Senator Arlen Specter's recent announcement of his switch in party affiliation, from Republican to Democrat, represents an important moment in Obama’s administration as well as a blow to the GOP. But Specter is hardly the first senator to cross party lines completely. In fact, Specter’s switch marks the 21^st time asenator has changed parties while serving since 1890.

Here, we take a look at some of the more notable examples.