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Tim Johnson latest to depart Senate

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Last November, just a couple of weeks after the 2012 elections, Senate Democratic leaders set a key electoral goal: convince Sens. Jay Rockefeller, Tim Johnson, Frank Lautenberg, Tom Harkin, and Carl Levin not to retire.

As of today, all five are departing the Senate at the end of this Congress.

South Dakota Sen. Tim Johnson will not seek re-election in 2014, NBC News has confirmed. He is expected to make a formal announcement of his retirement Tuesday.

Johnson's decision opens an opportunity for Republicans to take over the seat in a state that favored Mitt Romney over Barack Obama by almost 20 points.

His departure is no great surprise to operatives on both sides of the aisle. Johnson has been recovering from a brain hemorrhage in 2006 that left him partially paralyzed. (He did, however, run successfully for re-election in 2008.)

There are a couple of angles to consider in response to the news. The first is electoral -- Republicans need a net gain of six seats next year to take back the Senate majority, and Johnson's retirement clearly makes that task easier. Indeed, the GOP already has a top-tier candidate in the race: former Gov. Mike Rounds. Among South Dakota Democrats, meanwhile, expect former Rep. Stephanie Herseth Sandlin's phone to be ringing quite a bit today, along with U.S. Attorney Brendan Johnson, the incumbent senator's son.

In the larger context, despite the increasing radicalization of Republican politics and the party's national unpopularity, it's plausible to think the GOP can make significant Senate strides in 2014. A net gain of six is a tall order, but it's hardly a stretch to think Republicans can win open-seat contests in "red" states (South Dakota, West Virginia) and make a serious run at some Democratic incumbents in "red" states (Louisiana, North Carolina, Alaska, Arkansas).

The second area of interest is more of a big-picture question: is the Senate no longer a fun place to work?

Here we are in March, the year before the elections, and seven incumbent senators have already announced their retirement -- five Democrats and two Republicans. That's not a record, at least not yet, but it's a fairly large number at this stage in the cycle.

Which leads us to questions we kicked around a few weeks ago: What's driving this? Why are so many senators, most of whom would have been re-elected if they chose to run again, leaving their exclusive club?

We can only speculate, of course, but I have to wonder whether being a senator is a whole lot less fun that it used to be -- the chamber just isn't a satisfying place to be right now. Republican abuse of filibuster rules have made it nearly impossible to do much of anything, and given the extremism of the House majority, even if the Senate can pass something, it's likely to die in the lower chamber.

I'm also curious about the impact on the Senate itself of turnover on this scale. Consider the numbers since 2009 (not counting folks like Paul Kirk and Carte Goodwin who briefly filled temporary vacancies):

*111th Congress (2009-2010): 8 retirements, 5 resignations, 2 deaths, 2 defeats, 2 primary defeats

* 112th Congress (2011-2012): 10 retirements, 1 defeat, 1 resignation, 1 primary defeat

* 113th Congress (2013-2014): 7 retirements, 1 resignation, 1 death

Taken together, we're looking at a Senate poised to lose at least 41 of its members since 2009 -- and 24 of those 41 are voluntary retirements.

That's a level of turnover I might expect to see for a store at the mall, not in the U.S. Senate.