The Three Classes of the United States Senate

The United States Senate isn't a school per se, nor is it a rigid society with different levels of prestige.  Nevertheless, it does have classes...although they pertain neither to education nor social stratification.  There are three of them to be exact, and in a radical departure from typical government gobbledygook, they are simply designated as "1", "2", and "3".  But I haven't yet explained what these classes are, have I?  Well, continue on then, intrepid reader...

Every two years for our national Congress, we have elections for both the House of Representatives and the Senate.  Because all of the seats in the House are for only two years, then every one of them is up for election every two years (although a lot of them aren't contested, sad to say).  U.S. Senate terms, on the other hand, are for six years, and since there are a total of 100 senators...with 2 per state, their elections have been divided up into threes, with approximately a third of the seats up for election every two years.  So, depending on which of the three "classes" a senator belongs to, he or she will either bypass the congressional elections in a particular year or have to contend with a reelection campaign (if they still want to remain in the Senate, that is).  So, how do the classes specifically work?

In recent history, Class 1 senators (33 in all) have been elected in 2000, 2006, and 2012...and will be due for the next election in 2018.  Class 2 senators (33 in all) were elected in 2002, 2008, and 2014...and will be due for their next election in 2020.  And Class 3 senators (34 in all) were elected in 2004 and 2010...and will be up for election this year, 2016.  My home state of Florida has Classes 1 and 3...Marco Rubio's seat (he isn't running for reelection) is the Class 3 one that Floridians will decide on this November  (Democrat Bill Nelson is in Class 1).  Now class is important in Senate turnover and party control, because each class contains a certain number of Republican vs. Democratic senators.  According to Wikipedia, Class 1 contains 25 senators aligning with the Democrats and only 8 going for the Republican Party.  Class 2 is reversed, with 22 Republicans and 11 Democrats in it.  And the upcoming election in a few months is all about Class 3, with 24 Republican seats and only 10 Democratic that are up for a vote.  That means that, if you only look at the numbers, the Republicans have quite a task ahead of them this year if they want to retain the 54-46 majority they currently enjoy in the Senate.  And looking closer, some of the contested seats currently held by Republicans appear to be rather tenuous, especially in Wisconsin, Pennsylvania, Ohio, Illinois, and New Hampshire...