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Will Senate Democrats get their commeupance? We'll find out after 2016

What goes around, comes around.

Now that Senate Democrats changed the filibuster rules so that all executive presidential appointments -- save for those to the U.S. Supreme Court -- need a simple majority to win eventual confirmation, Republicans have vowed revenge.

Sen. Chuck Grassley, R-Iowa, even said Republicans would be eyeing the Supreme Court.

"So if the Democrats are bent on changing the rules, then I say go ahead," Grassley said earlier this month. "There are a lot more Scalias and [Clarence] Thomases that we'd love to put on the bench."

But for Republicans to accomplish that feat, they will need to do two things come 2016: 1) win the White House and 2) control the U.S. Senate.

As it turns out, Republicans are poised to make Senate gains in the 2014 midterms -- and maybe even win control of the chamber -- due to the favorable battleground. The key Senate contests are in red states like Alaska, Arkansas, Louisiana, Montana, North Carolina, South Dakota and West Virginia.

And to net the six Senate seats needed to retake the Senate, Republicans have to win all but one of these contests. It's a doable -- but narrow -- path.

But 2016 is a different story.

In that '16 cycle -- a presidential year -- Republicans will have to defend more Senate seats (24) than Democrats will (10). What's more, the turf is on more Democratic ground. Republicans up for re-election in 2016 include Illinois' Mark Kirk, Wisconsin's Ron Johnson, New Hampshire's Kelly Ayotte, Florida's Marco Rubio, and Pennsylvania's Pat Toomey.

Those are all states President Barack Obama won in both 2008 and 2012.