Another seat in the U.S. Senate will be in play in the 2014 mid-term elections, as Sen. Tim Johnson (D-SD) will reportedly announce on Tuesday that he is not running for re-election, presenting Republicans with a prime opportunity to pick up a seat in the Senate, and giving Democrats one more seat to defend.

So far, seven U.S. Senators have decided not to run for re-election; that's compared to 10 Senators who opted against a re-election bid in 2012 and eight Senators in 2010. Only five Senators retired before the 2008 elections.

As the numbers show, the 2014 cycle for Senators is already very active when it comes to retirements.

Here is the list of Senate departures so far in the 113th Congress:

+ Sen. Saxby Chambliss R-GA
+ Sen. Mike Johanns R-NE
+ Sen. Jay Rockefeller D-WV
+ Sen. Tom Harkin D-IA
+ Sen. Frank Lautenberg D-NJ
+ Sen. Carl Levin D-MI
+ Sen. Tim Johnson D-SD

On its face, the South Dakota Senate seat would seem to be a prime pickup opportunity for Republicans, a Red state that has voted reliability for the GOP in Presidential elections.

But that's what Republicans thought last year when a seat in North Dakota was up. That's what the GOP thought in Missouri in 2012.

And the GOP Trail of Tears in recent Senate elections extends much further than that, with head shaking losses in 2010 in Nevada, Colorado and Delaware.

Republicans probably have a chance to truly compete for open Senate seats in West Virginia, Iowa, South Dakota and Michigan; but New Jersey in 2014 would seem to be a longshot.

As for Johnson, there has been speculation for several years that he would not run for re-election, after suffering a stroke in late 2006 which left his speech slurred and forced him to use a wheelchair.

I wrote a blog about five weeks ago about how many Senate seats have changed hands in the last six years.

With Johnson's departure, that number grows again - to 45.

45 percent of the Senate will be different in January 2015 compared to how things were in 2008.

And there's still 22 months until we get to the new Congress, so that number should go up, whether by retirements or Senators actually defeated at the ballot box.

More change in the U.S. Senate.