This is the final of three Election Primers to help you get ready for Election Day.  To read our earlier post on the Presidential race, click here.  For the House of Representatives, click here.

The Senate – The Basics

There are 100 Senators, two from each of the 50 states.  Senators serve six-year terms, so only one third of the Senate seats are up in any given election year (barring a special election to fill a vacancy).

Because of a complicated procedural tactic known as the filibuster, most bills require 60 votes to receive cloture and be considered and passed in the Senate.  The filibuster and cloture will I’m sure be future topics for dewonkification, but for now it is simply important to realize that most legislation must reach the 60 vote threshold in order to receive an up-or-down vote.

The Vice President serves as the official President of the Senate, which in practice means that he acts as a tiebreaking vote in the event of a 50-50 split on matters requiring a simple majority (such as determining a Majority Leader).  So in the event of an evenly split chamber, the Vice President’s party ends up with a de facto majority.

The Senate currently has 51 Democrats, 47 Republicans, and two independents.  Both of those Independents (Senator Joseph Lieberman of Connecticut and Senator Bernie Sanders of Vermont) caucus with the Democrats, giving the party a functional majority of 53 seats.

2012 Election Outlook

Thirty-three Senate seats are up for grabs this year.  Of those, 23 are seats currently held by Democrats, and only ten are held by Republicans.  This makes the prospect of a net Republican gain likely.

How significant of a gain, however, is very much up in the air.  Currently, based on sitting Senators and poll projections, the Cook Political Report projects that 46 seats are likely to be Democratic, and 44 are likely to be Republican.  That leaves fully ten “toss-up” states where either party could win.  These include Arizona, Connecticut, Indiana, Maine, Massachusetts, Montana, Nevada, North Dakota, Virginia, and Wisconsin.

How these ten seats fall on Election Night will determine the balance of the Senate for the next two years.  Republicans would need to win seven of these ten seats to take the Senate if they do not also take the White House.  If Governor Mitt Romney were to win the presidency, they would need only six, with then-Vice President Paul Ryan casting the tiebreaking vote in the GOP’s favor.

But unless there is an unexpected landslide by either party, neither will achieve the 60 votes necessary to break a filibuster, meaning the agenda of whoever takes the majority will face the same roadblocks and stalls that have created gridlock in the chamber in recent years.

If you want to track the latest on the battle for the Senate, you can check out the Cook Political Report, Real Clear Politics, FiveThirtyEight, Politico, or Public Policy Polling, among many others.