Arlen Specter is not the first Senator to switch political parties and he will not be the last. Only three short years ago, Senator Joseph Lieberman of Connecticut switched from being a Democrat to an Independent Democrat, which means that he considers himself an Independent but belongs to the Democratic caucus.

Before Lieberman, in 2001, Senator James Jeffords of Vermont switched from Republican to Independent, and, like Lieberman, he sat on the Democratic side of the aisle. Obviously, there is no Independent aisle -- yet. Maybe it's time to consider squeezing one in.

Many people believe that Specter switched sides in an effort to escape the sinking ship known as the Republican Party. Even the ubiquitous "Joe the Plumber" recently stated that he doesn’t want to be a Republican anymore. During the Clinton years, two senators -- Richard Shelby of Alabama and Ben Nighthorse Campbell of Colorado -- also left their parties, thinking the grass would be greener on the other side of the aisle. They jumped the minority Democrat ship to join up with the majority Republicans. Another Clinton-era senator traveled a more complicated path.

In 1999, Robert Smith, a Republican Senator from New Hampshire, decided to run for the presidency. He quickly saw that he did not have a chance against the Republican front-runner, George W. Bush, so he ran as the candidate of the Taxpayers Party. When that effort fizzled, he changed his affiliation to Independent. Finally, he switched back to being a Republican and endorsed George W. Bush. The most notable aspect of Smith’s political party-hop is that he accomplished it all in just four months, between July and November of 1999.

Before Smith, Harry F. Byrd, Jr. of Virginia switched from Democrat to Independent. And some senators felt the need to sample everything on the political table: Strom Thurmond of South Carolina and Wayne Morse of Oregon each hold the distinction of having been, at one time, a Democrat, an Independent, and a Republican.

Going back to earlier days, there are several senators, including both Robert La Follette Sr. and Jr. of Wisconsin, who switched from the Republican to the short-lived Progressive Party. And back when the remonetization of silver was an issue, seven senators switched their affiliation from Republican to another party that fell by the wayside: Silver Republican.

Some senators switch parties in order to join one that more closely matches their beliefs. Some switch to enhance their electability. Many switch for a combination of both reasons. Of the eighteen senators mentioned in this article, seven survived their party-hopping to be elected to additional terms of office in their final party of choice. Nine did not. The fates of the two most recent party changers -- Lieberman and Specter -- remain to be seen.